feat(patreon): dead-letter ledger for permanently-failing media — #705 step 2 (#7)

A media that fails every walk (404'd CDN, deleted post, geo-blocked Mux,
persistently-corrupt bytes) used to re-error forever and re-burn chunks.
New `patreon_failed_media` table (alembic 0038, chains 0037) records
per-media attempts; once attempts reach DEAD_LETTER_THRESHOLD (3) the
ingester skips it on routine tick/backfill walks (tier-1.5, folded into the
seen/skip predicate). Recovery BYPASSES it (the operator's "try everything
again" re-attempts dead media). A clean download clears the row (recovered);
errors/quarantines upsert-increment it. Surfaced as
run_stats.dead_lettered_count.

- New PatreonFailedMedia model + migration; ingester _dead_keys /
  _record_failures (on_conflict increment) / _clear_failures.
- skip = seen | dead (empty in recovery); failures recorded post-fetch on
  short sessions (same pattern as the seen-ledger).

Tests: a media erroring 3× is dead-lettered + skipped (no download attempt);
recovery re-attempts a dead media and clears it on success; a clean download
clears a sub-threshold failure.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

alembic/versions/0038_patreon_failed_media.py

@@ -0,0 +1,58 @@
+"""patreon_failed_media: per-source dead-letter ledger for failing Patreon media
+
+Revision ID: 0038
+Revises: 0037
+Create Date: 2026-06-06
+
+Plan #705 (#7). Media that keeps failing to download/validate (404'd CDN,
+deleted post, geo-blocked Mux, persistently-corrupt bytes) gets recorded here
+with an attempt counter; once it crosses the dead-letter threshold the ingester
+skips it on routine walks (recovery still re-attempts). A clean download clears
+the row. UNIQUE (source_id, filehash) is the upsert key (same media key the
+seen-ledger uses).
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0038"
+down_revision: Union[str, None] = "0037"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "patreon_failed_media",
+        sa.Column("id", sa.Integer, primary_key=True),
+        sa.Column(
+            "source_id",
+            sa.Integer,
+            sa.ForeignKey("source.id", ondelete="CASCADE"),
+            nullable=False,
+            index=True,
+        ),
+        sa.Column("filehash", sa.String(128), nullable=False),
+        sa.Column("attempts", sa.Integer, nullable=False, server_default="1"),
+        sa.Column("last_error", sa.Text, nullable=True),
+        sa.Column(
+            "first_failed_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.text("NOW()"),
+        ),
+        sa.Column(
+            "last_failed_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.text("NOW()"),
+        ),
+        sa.UniqueConstraint(
+            "source_id", "filehash", name="uq_patreon_failed_media_source_id"
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("patreon_failed_media")