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>

backend/app/services/link_extract.py

@@ -0,0 +1,105 @@
+"""Extract off-platform file-host links from a post body.
+
+Pure (no I/O) so it's unit-testable and reusable by EVERY in-house downloader's
+import path — every platform stores its body in `Post.description`, so running
+this there covers them all. Finds links to the supported external file hosts in
+a post's HTML body (both `<a href="...">` anchors and bare URLs in text),
+unwraps a Patreon outbound-redirect wrapper, and preserves the FULL url
+including the `#fragment` (mega.nz puts the decryption key there) and the query
+string — without those a mega download is impossible.
+"""
+from __future__ import annotations
+
+import re
+from dataclasses import dataclass
+from html import unescape
+from urllib.parse import parse_qs, urlsplit
+
+# Supported file-host enum values (kept in sync with the external_link CHECK).
+SUPPORTED_HOSTS = ("mega", "gdrive", "mediafire", "dropbox", "pixeldrain")
+
+# Bare-domain suffix → canonical host label. Matched against the URL netloc
+# (exact or as a dotted suffix, so www./dl. subdomains resolve too).
+_HOST_MAP = {
+    "mega.nz": "mega",
+    "mega.co.nz": "mega",
+    "drive.google.com": "gdrive",
+    "mediafire.com": "mediafire",
+    "dropbox.com": "dropbox",
+    "pixeldrain.com": "pixeldrain",
+}
+
+# `<a href="...">label</a>` — DOTALL so a label spanning tags/newlines is caught.
+_HREF_RE = re.compile(
+    r"""<a\b[^>]*?\bhref=["']([^"']+)["'][^>]*>(.*?)</a>""",
+    re.IGNORECASE | re.DOTALL,
+)
+_TAG_RE = re.compile(r"<[^>]+>")
+# Bare http(s) URL in text. Stops at whitespace, quotes, angle brackets, and
+# closing brackets — but KEEPS `#`, `&`, `?`, `=` so fragments/queries survive.
+_URL_RE = re.compile(r"""https?://[^\s"'<>)\]}]+""", re.IGNORECASE)
+
+
+@dataclass(frozen=True)
+class ExtractedLink:
+    host: str            # one of SUPPORTED_HOSTS
+    url: str             # full url incl. query + #fragment
+    label: str | None    # visible anchor text, when present
+
+
+def _netloc(url: str) -> str:
+    # Lowercase host without credentials or port.
+    return urlsplit(url).netloc.lower().split("@")[-1].split(":")[0]
+
+
+def host_for(url: str) -> str | None:
+    """Canonical host label for a url, or None if it's not a supported host."""
+    netloc = _netloc(url)
+    for suffix, host in _HOST_MAP.items():
+        if netloc == suffix or netloc.endswith("." + suffix):
+            return host
+    return None
+
+
+def _unwrap(url: str) -> str:
+    """Unwrap a Patreon outbound-redirect wrapper to its inner target, so a
+    wrapped mega/gdrive link resolves to the real host. Patreon has used both
+    `www.patreon.com/...?url=<encoded>` and the `l.patreon.com` shim; check the
+    common target-param names. Non-Patreon urls pass through untouched."""
+    netloc = _netloc(url)
+    if not (netloc == "patreon.com" or netloc.endswith(".patreon.com")):
+        return url
+    qs = parse_qs(urlsplit(url).query)
+    for key in ("url", "u", "ext_url", "redirect", "target"):
+        vals = qs.get(key)
+        if vals and vals[0]:
+            return unescape(vals[0]).strip()
+    return url
+
+
+def extract_external_links(html: str | None) -> list[ExtractedLink]:
+    """All supported-host links in `html`, de-duplicated by url (first wins, so
+    an anchor's label is kept over a later bare sighting of the same url)."""
+    if not html:
+        return []
+    found: dict[str, ExtractedLink] = {}
+
+    # 1) Anchors first — they carry a human label ("Mega - Streamable").
+    for raw_href, inner in _HREF_RE.findall(html):
+        url = _unwrap(unescape(raw_href).strip())
+        host = host_for(url)
+        if host is None:
+            continue
+        label = unescape(_TAG_RE.sub("", inner)).strip() or None
+        found.setdefault(url, ExtractedLink(host=host, url=url, label=label))
+
+    # 2) Bare URLs pasted in text (no label). Trailing prose punctuation is
+    #    trimmed; the href values already captured above de-dup away here.
+    for raw in _URL_RE.findall(html):
+        url = _unwrap(unescape(raw).strip().rstrip(".,;"))
+        host = host_for(url)
+        if host is None:
+            continue
+        found.setdefault(url, ExtractedLink(host=host, url=url, label=None))
+
+    return list(found.values())