chore: audit-flagged cleanups (no behaviour change)

Four small follow-ups from the 0.9.0 audit, none of them
user-visible:

- Migration 0007 drops `chapters_manga_idx`. The 0001 schema declared
  both `UNIQUE (manga_id, number)` and `CREATE INDEX chapters_manga_idx
  ON (manga_id, number)`, but Postgres maintains an identical index
  for the unique constraint automatically — the explicit one was just
  paying for a second per-write update. Query plans are unchanged
  because the planner already preferred the constraint's index.
- `upload::parse_image` sniffs from the first 64 bytes instead of the
  full image buffer. `infer` only looks at magic bytes anyway, so
  scanning 20 MiB is wasted work. Functionally identical; cheaper in
  the hot path.
- AVIF was on the whitelist but had no test fixture. New `avif_bytes`
  helper produces a minimal `ftyp avif` header that `infer` recognises,
  and a new `accepts_avif` unit test covers the path end-to-end.
- Frontend `request()` sets `credentials: 'include'`. Same-origin
  callers see no change (default was already `'same-origin'`), but the
  first user who configures `CORS_ALLOWED_ORIGINS` for a cross-origin
  deployment gets working cookies without having to chase a runtime
  ApiError trail.

No version bump.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/migrations/0007_drop_redundant_chapter_index.sql

@@ -0,0 +1,7 @@
+-- `chapters_manga_idx` over (manga_id, number) duplicates the implicit
+-- index Postgres maintains for the `UNIQUE (manga_id, number)`
+-- constraint from 0001 — same leading columns, same ordering, same
+-- selectivity. Dropping the explicit one saves a per-write index
+-- update without changing query plans.
+
+DROP INDEX IF EXISTS chapters_manga_idx;