feat(posts): extract + record external file-host links (Phase 3)

Capture off-platform links (mega/gdrive/mediafire/dropbox/pixeldrain) embedded
in post bodies so they're never silently dropped, and surface them in the post
view. The download worker (Phase 4) walks these rows.

- link_extract.py: pure extractor — <a href> + bare URLs, unwraps Patreon
  redirect shims, PRESERVES the full url incl. #fragment (mega's key), dedups.
  Reusable by every platform (runs off Post.description).
- external_link model + migration 0049: post_id/artist_id/host/url/label/status
  /attempts/last_error/attachment_id/timing; CHECK whitelists (full enum incl.
  worker statuses up front) + (post_id,url) unique.
- importer._sync_external_links: insert-missing on both import paths
  (_apply_sidecar + upsert_post_record) so a re-import never resets a link's
  status; runs for all platforms.
- post_feed_service.get_post: returns external_links (detail-only).
- PostCard: renders the links (host chip + label + status) once expanded.
- tests: extractor (5 hosts, fragment, shim unwrap, dedup), importer (record +
  no-dup on reimport), serializer.

Refs FC #830.

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

alembic/versions/0049_external_link_table.py

@@ -0,0 +1,90 @@
+"""external_link table — off-platform file-host links found in post bodies
+
+Creators host the real files on mega.nz / Google Drive / MediaFire / Dropbox /
+Pixeldrain and link them in the post text. This table records each such link
+(so nothing is silently dropped), and doubles as the dedup + dead-letter ledger
+the download worker (a later slice) walks. `url` keeps the FULL link including
+the `#fragment` — mega.nz's decryption key lives there; truncating it makes the
+file undownloadable.
+
+CHECK whitelists for host + status include the full enum up front (incl. the
+download-worker statuses) so the worker slice needs no constraint migration.
+
+Revision ID: 0049
+Revises: 0048
+Create Date: 2026-06-14
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0049"
+down_revision: Union[str, None] = "0048"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "external_link",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "post_id", sa.Integer(),
+            sa.ForeignKey("post.id", ondelete="CASCADE"), nullable=False,
+        ),
+        sa.Column(
+            "artist_id", sa.Integer(),
+            sa.ForeignKey("artist.id", ondelete="SET NULL"), nullable=True,
+        ),
+        sa.Column("host", sa.String(length=16), nullable=False),
+        sa.Column("url", sa.Text(), nullable=False),
+        sa.Column("label", sa.Text(), nullable=True),
+        sa.Column(
+            "status", sa.String(length=16), nullable=False,
+            server_default="pending",
+        ),
+        sa.Column("attempts", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("last_error", sa.Text(), nullable=True),
+        sa.Column(
+            "attachment_id", sa.Integer(),
+            sa.ForeignKey("post_attachment.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+        sa.Column(
+            "created_at", sa.DateTime(timezone=True), nullable=False,
+            server_default=sa.func.now(),
+        ),
+        sa.Column("completed_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("duration_seconds", sa.Float(), nullable=True),
+        sa.CheckConstraint(
+            "host IN ('mega','gdrive','mediafire','dropbox','pixeldrain')",
+            name="ck_external_link_host",
+        ),
+        sa.CheckConstraint(
+            "status IN ('pending','downloading','downloaded','failed',"
+            "'skipped','dead')",
+            name="ck_external_link_status",
+        ),
+    )
+    op.create_index(
+        "ix_external_link_post_id", "external_link", ["post_id"],
+    )
+    op.create_index(
+        "ix_external_link_artist_id", "external_link", ["artist_id"],
+    )
+    op.create_index(
+        "ix_external_link_status", "external_link", ["status"],
+    )
+    op.create_index(
+        "uq_external_link_post_url", "external_link", ["post_id", "url"],
+        unique=True,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("uq_external_link_post_url", table_name="external_link")
+    op.drop_index("ix_external_link_status", table_name="external_link")
+    op.drop_index("ix_external_link_artist_id", table_name="external_link")
+    op.drop_index("ix_external_link_post_id", table_name="external_link")
+    op.drop_table("external_link")