Verify access on progress updates; add S16/S17/S18 scenarios

Bug fix: POST /api/v1/progress and POST /api/v1/progress/batch did not
verify that the supplied media_id belonged to a media row the caller
could see. Two failure modes:

  - Missing media_id triggered an FK violation in UpsertProgress, which
    fell through handleError to HTTP 500 instead of 404.
  - Soft-deleted media_id (row still exists, deleted_at != nil) was
    accepted silently with HTTP 200, recording progress on an item the
    user could no longer reach.

Both now route through accessHelper.verifyAccess in progressService,
which returns ErrNotFound (404) for missing/soft-deleted rows and
ErrForbidden (403) for unauthorized sets. Verified via curl:
POST /progress media_id=999999999 → 404; POST /progress/batch with a
bad id → 404.

Tests: progress_test.go and no_rows_test.go now seed MediaRepo and
UserRepo so the verifyAccess branch finds a real (admin) caller and a
real media row. All other tests untouched.

S16 covers media list pagination, filtering, sort, and the parser's
intentional fail-open behaviour for malformed limit/offset/type.
S17 covers podcast list endpoints (GET /podcasts, GET /podcasts/{id}/
episodes) and the admin-only subscribe gate. S18 covers single
POST /progress + GET /in-progress, including the new 404 path for
missing/forbidden media. Full LLM e2e suite passes 18/18.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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