feat: require two recent failures to queue a season

The season branch of the attention queue listed a season on one failed
grab of any age, so `GET /api/queues/attention` returned Rick and Morty
with every season it has and buried the one that needed attention.

Two changes, both stated in DESIGN.md §5.7:

- The season branch now enforces the same bar the episode branch does:
  two grabs that hard-failed on *different* releases.
- A failed grab counts toward the queue for 30 days
  (`arr_db::ATTENTION_WINDOW`). Nothing clears a `grabs` row, so without
  a window the queue only grows and can never be emptied. #181 gave the
  pack guard a backoff curve for the same reason; this is the queue's
  version of §6.2's "it never gives up entirely, it goes quiet". A
  season the operator dealt with stops failing and drops out; one still
  breaking keeps failing (the pack guard retries at worst weekly) and
  stays.

The window applies to all three hard-fail lanes — movie, episode and
season — because DESIGN.md states one rule for the queue, and to the
daemon's needs-a-decision notifier as well as the API, since both read
the same queue and a season-per-failure notification is the same noise
on a different channel. No schema change: `grabs.grabbed_at` already
carries the timestamp.

Gate: `just ci` green (486 tests).

crates/arr-db/src/lib.rs

@@ -17,6 +17,15 @@ use sqlx::{migrate::MigrateError, SqlitePool};
 /// The migrations embedded in the binary, so a deploy is one file.
 pub static MIGRATOR: sqlx::migrate::Migrator = sqlx::migrate!("./migrations");
 
+/// §5.7: how long a failed grab keeps counting toward the needs-a-decision
+/// queue, as a SQLite time modifier.
+///
+/// Nothing ever clears a `grabs` row, so without a bound the queue only grows
+/// and the one season that wants attention sits behind the ones that do not.
+/// Callers pair it with the `grabbed_at` format:
+/// `strftime('%Y-%m-%dT%H:%M:%fZ', 'now', ATTENTION_WINDOW)`.
+pub const ATTENTION_WINDOW: &str = "-30 days";
+
 /// How long a writer waits for the write lock before giving up.
 const BUSY_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(5);