feat(subs): translate from an image-only release

A release whose only subtitle is a PGS or VobSub track was stuck both
ways: the track satisfied English so no English SRT was ever fetched,
and bitmaps can never feed a translator. §15 is amended to separate
satisfying viewing from providing a translation source, and to carve a
fetch made to obtain a source out of the no-upgrade rule.

Timings come from the disc rather than from alass guessing at the audio.
At import, each non-forced image track's packet timestamps are paired
show-to-clear into a cue skeleton and stored; the fetched source is then
aligned against that skeleton, and the translation made from it skips
the post-translation pass, which could only move disc-exact timings off.

Pairing is validated before it is trusted — even packet count, plausible
durations, sane density for the runtime — because PGS allows several
composition segments per subtitle and a slipped pairing is quietly half
a second out. A track that fails validation gets no skeleton and falls
back to aligning against the video, as does any file imported before
this: there is no backfill.

Closes #268

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.sqlx/query-266c8e63e7f81cb07921de180f32b47ac8c4e9d47a31af786742404f8e54f517.json

@@ -1,6 +1,6 @@
 {
   "db_name": "SQLite",
-  "query": "INSERT INTO subtitle_files\n               (media_file_id, language, origin, provider, candidate_id, engine,\n                forced, sdh, synced, sync_rejected, path)\n           VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)\n           ON CONFLICT (path) DO NOTHING\n           ON CONFLICT (media_file_id, language, forced, sdh)\n               WHERE origin = 'embedded' DO NOTHING\n           RETURNING id AS \"id!: i64\"",
+  "query": "INSERT INTO subtitle_files\n               (media_file_id, language, origin, provider, candidate_id, engine,\n                forced, sdh, synced, sync_rejected, skeleton_aligned, path)\n           VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)\n           ON CONFLICT (path) DO NOTHING\n           ON CONFLICT (media_file_id, language, forced, sdh)\n               WHERE origin = 'embedded' DO NOTHING\n           RETURNING id AS \"id!: i64\"",
   "describe": {
     "columns": [
       {
@@ -16,11 +16,11 @@
       }
     ],
     "parameters": {
-      "Right": 11
+      "Right": 12
     },
     "nullable": [
       null
     ]
   },
-  "hash": "3d48b78768bf8dc2e337d0935889a5654089f38bb7ee2a15b7e19f4ed5540262"
+  "hash": "266c8e63e7f81cb07921de180f32b47ac8c4e9d47a31af786742404f8e54f517"
 }

.sqlx/query-8651688f9f999629b4996c0451f87f5a84fabe748849b4b80063af1c18f73917.json

@@ -0,0 +1,26 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT id AS \"id!: i64\" FROM media_files WHERE path = ?",
+  "describe": {
+    "columns": [
+      {
+        "name": "id!: i64",
+        "ordinal": 0,
+        "type_info": "Integer",
+        "origin": {
+          "Table": {
+            "table": "media_files",
+            "name": "id"
+          }
+        }
+      }
+    ],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": [
+      false
+    ]
+  },
+  "hash": "8651688f9f999629b4996c0451f87f5a84fabe748849b4b80063af1c18f73917"
+}

.sqlx/query-8abb1e992e0972a19e76c7788c8c350951a1d14766088b1cf3fb886fb658d575.json

@@ -1,6 +1,6 @@
 {
   "db_name": "SQLite",
-  "query": "SELECT id            AS \"id!: i64\",\n                  media_file_id AS \"media_file_id!: i64\",\n                  language      AS \"language!: String\",\n                  origin        AS \"origin!: SubtitleOrigin\",\n                  provider,\n                  candidate_id,\n                  engine,\n                  forced        AS \"forced!: bool\",\n                  sdh           AS \"sdh!: bool\",\n                  synced        AS \"synced!: bool\",\n                  sync_rejected AS \"sync_rejected!: bool\",\n                  path\n           FROM   subtitle_files\n           WHERE  media_file_id = ?\n           ORDER BY language, id",
+  "query": "SELECT id            AS \"id!: i64\",\n                  media_file_id AS \"media_file_id!: i64\",\n                  language      AS \"language!: String\",\n                  origin        AS \"origin!: SubtitleOrigin\",\n                  provider,\n                  candidate_id,\n                  engine,\n                  forced        AS \"forced!: bool\",\n                  sdh           AS \"sdh!: bool\",\n                  synced        AS \"synced!: bool\",\n                  sync_rejected AS \"sync_rejected!: bool\",\n                  skeleton_aligned AS \"skeleton_aligned!: bool\",\n                  path\n           FROM   subtitle_files\n           WHERE  media_file_id = ?\n           ORDER BY language, id",
   "describe": {
     "columns": [
       {
@@ -125,8 +125,19 @@
         }
       },
       {
-        "name": "path",
+        "name": "skeleton_aligned!: bool",
         "ordinal": 11,
+        "type_info": "Integer",
+        "origin": {
+          "Table": {
+            "table": "subtitle_files",
+            "name": "skeleton_aligned"
+          }
+        }
+      },
+      {
+        "name": "path",
+        "ordinal": 12,
         "type_info": "Text",
         "origin": {
           "Table": {
@@ -151,8 +162,9 @@
       false,
       false,
       false,
+      false,
       true
     ]
   },
-  "hash": "62d625c322c8c64b48317cdbf466d74043708d8e12997e0efec23a18eaaaaace"
+  "hash": "8abb1e992e0972a19e76c7788c8c350951a1d14766088b1cf3fb886fb658d575"
 }

.sqlx/query-8e3c23ed488bc82ff3dd4f6da02d00435525d321b6002b9f31979a39281369ce.json

@@ -0,0 +1,12 @@
+{
+  "db_name": "SQLite",
+  "query": "INSERT INTO subtitle_skeletons (media_file_id, stream_index, language, cues)\n         VALUES (?, ?, ?, ?)\n         ON CONFLICT (media_file_id, stream_index) DO UPDATE SET\n             language = excluded.language,\n             cues     = excluded.cues",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Right": 4
+    },
+    "nullable": []
+  },
+  "hash": "8e3c23ed488bc82ff3dd4f6da02d00435525d321b6002b9f31979a39281369ce"
+}

.sqlx/query-eb7df52055a60a00b721e3eef152e1b04a8512a85e8ad3618545c634c23abf31.json

@@ -0,0 +1,50 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT stream_index AS \"stream_index!: i64\",\n                  language     AS \"language!: String\",\n                  cues         AS \"cues!: String\"\n           FROM   subtitle_skeletons\n           WHERE  media_file_id = ?\n           ORDER BY stream_index",
+  "describe": {
+    "columns": [
+      {
+        "name": "stream_index!: i64",
+        "ordinal": 0,
+        "type_info": "Integer",
+        "origin": {
+          "Table": {
+            "table": "subtitle_skeletons",
+            "name": "stream_index"
+          }
+        }
+      },
+      {
+        "name": "language!: String",
+        "ordinal": 1,
+        "type_info": "Text",
+        "origin": {
+          "Table": {
+            "table": "subtitle_skeletons",
+            "name": "language"
+          }
+        }
+      },
+      {
+        "name": "cues!: String",
+        "ordinal": 2,
+        "type_info": "Text",
+        "origin": {
+          "Table": {
+            "table": "subtitle_skeletons",
+            "name": "cues"
+          }
+        }
+      }
+    ],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": [
+      false,
+      false,
+      false
+    ]
+  },
+  "hash": "eb7df52055a60a00b721e3eef152e1b04a8512a85e8ad3618545c634c23abf31"
+}