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.

backend/app/utils/sidecar.py

@@ -55,6 +55,33 @@ def _first_str(data: dict, keys: tuple[str, ...]) -> str | None:
     return None
 
 
+# Strip HTML tags + collapse whitespace + take the first non-empty line.
+# Used to derive a display title from a body when the platform doesn't
+# expose a separate title field (subscribestar posts always write
+# `title: ""` and put the leading sentence inside `content` as HTML).
+# Truncated to 120 chars with an ellipsis if longer — long enough to be
+# meaningful in a feed, short enough to fit a row.
+_TAG_RE = re.compile(r"<[^>]+>")
+_WS_RE = re.compile(r"\s+")
+
+
+def _first_line_text(body: str, limit: int = 120) -> str | None:
+    if not body:
+        return None
+    text = _TAG_RE.sub(" ", body)
+    text = text.replace("\xa0", " ")
+    # Split on hard line breaks first; the body-stripped HTML often
+    # collapses to one logical line, in which case the first sentence
+    # split is the next-best heuristic.
+    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 _parse_date(v) -> datetime | None:
     if isinstance(v, bool):
         return None
@@ -111,12 +138,24 @@ def parse_sidecar(data: dict) -> SidecarData:
             if post_date is not None:
                 break
 
+    description = _first_str(data, ("content", "description", "caption"))
+
+    # SubscribeStar posts always write `title: ""` and put the leading
+    # sentence inside `content` (confirmed against the operator's
+    # /mnt/Data/Patreon/Cheunart/subscribestar/ dump 2026-05-27). When
+    # no explicit title is present, synthesize one from the description
+    # body's first non-empty line. Patreon retains its explicit titles
+    # because they're non-empty and short-circuit the fallback.
+    post_title = _first_str(data, ("title",))
+    if post_title is None and description:
+        post_title = _first_line_text(description)
+
     return SidecarData(
         platform=platform,
         external_post_id=external_post_id,
         post_url=_first_str(data, ("url", "post_url")),
-        post_title=_first_str(data, ("title",)),
-        description=_first_str(data, ("content", "description", "caption")),
+        post_title=post_title,
+        description=description,
         attachment_count=attachment_count,
         post_date=post_date,
         raw=data,