feat(import-resilience L1): poison-pill circuit breaker — cap stuck-task re-queues

Layer 1 of the import-task resilience work (operator-requested
2026-05-28). The recover_interrupted_tasks sweep re-queues rows stuck
in 'processing' — correct for a worker crash, but without a cap a row
that RELIABLY hard-crashes the worker (OOM/segfault/SIGKILL on a
corrupt or oversized input) loops forever: re-queue → crash → re-queue,
burning a worker slot every 5 min. A caught exception flips to terminal
'failed' and never enters this loop; only process-killing inputs do.

- alembic 0026: import_task.recovery_count (int, default 0) +
  import_task.refetched (bool, default false — backs Layer 2).
- recover_interrupted_tasks now runs a poison-pill UPDATE FIRST: stuck
  rows whose recovery_count has already reached MAX_RECOVERY_ATTEMPTS-1
  are marked 'failed' with a diagnostic ("crashed or stalled the worker
  N times … likely a corrupt or oversized input … inspect/replace the
  file, then retry via /api/import/retry-failed") instead of re-queued.
  The re-queue pass then handles the remaining stuck rows and bumps
  recovery_count. Shared stuck_predicate (and_/or_) keeps the
  media-5min / archive-40min split.
- MAX_RECOVERY_ATTEMPTS=3 (two recoveries then give up).

The failed poison pill surfaces in the existing import-failures view
with its file path, directly answering "help me identify them."

Test test_recover_interrupted_poison_pill_caps_at_max pins both
branches: a row at the cap is failed (not re-enqueued, diagnostic
present), a row one short is re-queued + incremented.

backend/app/models/import_task.py

@@ -8,7 +8,16 @@ been processing longer than the stuck-task threshold.
 
 from datetime import datetime
 
-from sqlalchemy import BigInteger, DateTime, ForeignKey, Integer, String, Text, func
+from sqlalchemy import (
+    BigInteger,
+    Boolean,
+    DateTime,
+    ForeignKey,
+    Integer,
+    String,
+    Text,
+    func,
+)
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from .base import Base
@@ -26,6 +35,13 @@ class ImportTask(Base):
     task_type: Mapped[str] = mapped_column(String(16), nullable=False)  # media|archive
     status: Mapped[str] = mapped_column(String(16), nullable=False, default="pending", index=True)
 
+    # Poison-pill circuit breaker (alembic 0026). recovery_count tracks
+    # how many times the stuck-task sweep has re-queued this row; after
+    # the cap it's failed with a diagnostic instead of looping. refetched
+    # bounds the one-shot re-download remediation to a single attempt.
+    recovery_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    refetched: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+
     result_image_id: Mapped[int | None] = mapped_column(
         ForeignKey("image_record.id", ondelete="SET NULL"), nullable=True
     )