feat(pixiv): ledger models + migration 0076 + PixivIngester adapter (#129 step 3)

pixiv_seen_media / pixiv_failed_media mirror the Patreon/SubscribeStar
ledgers (keys are always synthesized <illust_id>:p<num> / <illust_id>:ugoira
— pximg URLs carry no content hash). PixivIngester wires client/downloader/
ledgers into ingest_core with drift label 'Pixiv app API' and the new
body_canary=False opt-out: caption-less pixiv artists are common, so the
zero-bodies #862 alarm would false-positive here — the client's
response-shape drift checks cover that failure class instead. auth_token
joins the uniform adapter constructor (pixiv is the first token-auth native
platform). verify_pixiv_credential = one OAuth refresh, no feed walk.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/models/pixiv_seen_media.py

@@ -0,0 +1,42 @@
+"""PixivSeenMedia — per-source ledger of Pixiv media already
+downloaded+processed.
+
+Mirror of PatreonSeenMedia/SubscribeStarSeenMedia for the Pixiv native
+ingester (replacing gallery-dl). One queryable row per (source, media) so
+routine walks skip media we've already ingested; recovery mode bypasses the
+ledger to re-walk.
+
+Pixiv original URLs carry no content hash, so `filehash` is always the
+synthesized ``<illust_id>:p<num>`` (page) / ``<illust_id>:ugoira`` (frame
+zip) key — stable across any URL-shape drift. String(128) matches the sibling
+ledgers.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import ForeignKey, Integer, String, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.types import DateTime
+
+from .base import Base
+
+
+class PixivSeenMedia(Base):
+    __tablename__ = "pixiv_seen_media"
+    __table_args__ = (
+        # Dedup key the downloader upserts against: one ledger row per
+        # (source, media). A second sighting of the same media is a no-op.
+        UniqueConstraint(
+            "source_id", "filehash", name="uq_pixiv_seen_media_source_id"
+        ),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    source_id: Mapped[int] = mapped_column(
+        ForeignKey("source.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    filehash: Mapped[str] = mapped_column(String(128), nullable=False)
+    post_id: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    seen_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )