fix(sidecar): synthesize post_title from content first-line when title is empty (subscribestar)

SubscribeStar gallery-dl always writes `title: ""` and embeds the leading
sentence inside `content` HTML. Confirmed against the operator's
/mnt/Data/Patreon/Cheunart/subscribestar/ dump 2026-05-27: every
post's JSON has `title: ""` and a content like
`<div>Lets say hello to you guys with my Belle <br><br><br></div>`.
FC's sidecar parser, treating empty strings as missing, had been leaving
post_title NULL on every subscribestar post since FC-3 shipped.

Fix at two layers:

1. `backend/app/utils/sidecar.py` — new `_first_line_text(body, limit)`
   helper strips HTML tags, collapses whitespace, returns the first
   non-empty line truncated to 120 chars with ellipsis. `parse_sidecar`
   now falls back to this when `title` resolves to None and a
   `content`/`description`/`caption` value is present. Patreon's
   non-empty titles short-circuit the fallback so existing behavior is
   unchanged. Four new tests in test_sidecar_util.py pin: derivation
   from content, truncation at 120 chars, explicit-title precedence,
   no-content no-fallback.

2. `alembic 0024_backfill_post_title_from_description` — backfills the
   same logic across existing Post rows where `post_title IS NULL OR
   post_title = ''` AND description is present. Idempotent (re-running
   is a no-op once titles are populated). Downgrade is a no-op since
   there's no safe way to tell derived rows from genuine ones.

After deploy + migration: subscribestar posts will surface a meaningful
title in PostCard, post feed search, etc.

alembic/versions/0024_backfill_post_title_from_description.py

@@ -0,0 +1,80 @@
+"""backfill post.post_title from description first-line — 2026-05-27
+
+Revision ID: 0024
+Revises: 0023
+Create Date: 2026-05-27
+
+SubscribeStar gallery-dl always writes `title: ""` and embeds the leading
+sentence inside `content` HTML. FC's sidecar parser was leaving
+post_title NULL for every SubscribeStar post since FC-3 shipped. The
+parser fix (sidecar._first_line_text fallback) now synthesizes a title
+at parse time; this migration applies the same logic retroactively to
+existing rows.
+
+Operator-flagged 2026-05-27 after inspecting
+/mnt/Data/Patreon/Cheunart/subscribestar/ sidecars.
+
+Idempotent: only touches rows where post_title IS NULL or empty AND
+description IS NOT NULL. Re-running the migration is a no-op.
+"""
+from __future__ import annotations
+
+import re
+from typing import Sequence, Union
+
+from alembic import op
+from sqlalchemy import text
+
+revision: str = "0024"
+down_revision: Union[str, None] = "0023"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+_TAG_RE = re.compile(r"<[^>]+>")
+_WS_RE = re.compile(r"\s+")
+
+
+def _first_line_text(body: str, limit: int = 120) -> str | None:
+    """Mirror of sidecar._first_line_text. Kept inline so the migration
+    doesn't carry a runtime import dependency from app code that may
+    have moved by the time the migration is replayed years from now."""
+    if not body:
+        return None
+    text_ = _TAG_RE.sub(" ", body)
+    text_ = text_.replace("\xa0", " ")
+    for line in text_.splitlines():
+        line = _WS_RE.sub(" ", line).strip()
+        if line:
+            if len(line) > limit:
+                return line[: limit - 1].rstrip() + "…"
+            return line
+    return None
+
+
+def upgrade() -> None:
+    bind = op.get_bind()
+    rows = bind.execute(
+        text(
+            "SELECT id, description FROM post "
+            "WHERE (post_title IS NULL OR post_title = '') "
+            "AND description IS NOT NULL AND description <> ''"
+        )
+    ).fetchall()
+    updated = 0
+    for row in rows:
+        derived = _first_line_text(row.description)
+        if not derived:
+            continue
+        bind.execute(
+            text("UPDATE post SET post_title = :t WHERE id = :id"),
+            {"t": derived, "id": row.id},
+        )
+        updated += 1
+    print(f"0024: backfilled post_title on {updated} row(s)")
+
+
+def downgrade() -> None:
+    # No safe restore — we can't tell which post_titles were derived vs
+    # genuinely present. Leave the column alone on rollback.
+    pass