Add AppImageHub.com OCS API as primary catalog source

Integrate the Pling/OCS REST API (appimagehub.com) as the primary catalog
source with richer metadata than the existing appimage.github.io feed.

Backend:
- Add OCS API fetch with pagination, lenient JSON deserializers for loosely
  typed numeric fields, and non-AppImage file filtering (.dmg, .exe, etc.)
- Database migration v17 adds OCS-specific columns (ocs_id, downloads, score,
  typename, personid, description, summary, version, tags, etc.)
- Deduplicate secondary source apps against OCS entries
- Shrink OCS CDN icon URLs from 770x540 to 100x100 for faster loading
- Clear stale screenshot and icon caches on sync
- Extract GitHub repo links from OCS HTML descriptions
- Add fetch_ocs_download_files() to get all version files for an app
- Resolve fresh JWT download URLs per slot at install time

Detail page:
- Fetch OCS download files on page open and populate install SplitButton
  with version dropdown (newest first, filtered for AppImage only)
- Show OCS metadata: downloads, score, author, typename, tags, comments,
  created/updated dates, architecture, filename, file size, MD5
- Prefer ocs_description (full HTML with features/changelog) over short
  summary for the About section
- Add html_to_description() to preserve formatting (lists, paragraphs)
- Remove redundant Download link from Links section
- Escape ampersands in Pango markup subtitles (categories, typename, tags)

Catalog view:
- OCS source syncs first as primary, appimage.github.io as secondary
- Featured apps consider OCS download counts alongside GitHub stars

UI:
- Add pulldown-cmark for GitHub README markdown rendering in detail pages
- Add build_markdown_view() widget for rendered markdown content

Cargo.lock

@@ -559,6 +559,7 @@ dependencies = [
  "log",
  "notify",
  "notify-rust",
+ "pulldown-cmark",
  "quick-xml",
  "rusqlite",
  "serde",
@@ -876,6 +877,15 @@ dependencies = [
  "version_check",
 ]
 
+[[package]]
+name = "getopts"
+version = "0.2.24"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cfe4fbac503b8d1f88e6676011885f34b7174f46e59956bba534ba83abded4df"
+dependencies = [
+ "unicode-width",
+]
+
 [[package]]
 name = "getrandom"
 version = "0.2.17"
@@ -1804,6 +1814,25 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "pulldown-cmark"
+version = "0.12.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f86ba2052aebccc42cbbb3ed234b8b13ce76f75c3551a303cb2bcffcff12bb14"
+dependencies = [
+ "bitflags 2.11.0",
+ "getopts",
+ "memchr",
+ "pulldown-cmark-escape",
+ "unicase",
+]
+
+[[package]]
+name = "pulldown-cmark-escape"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "007d8adb5ddab6f8e3f491ac63566a7d5002cc7ed73901f72057943fa71ae1ae"
+
 [[package]]
 name = "quick-xml"
 version = "0.37.5"
@@ -2335,12 +2364,24 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "unicase"
+version = "2.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dbc4bc3a9f746d862c45cb89d705aa10f187bb96c76001afab07a0d35ce60142"
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.24"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e6e4313cd5fcd3dad5cafa179702e2b244f760991f45397d14d4ebf38247da75"
 
+[[package]]
+name = "unicode-width"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b4ac048d71ede7ee76d585517add45da530660ef4390e49b098733c6e897f254"
+
 [[package]]
 name = "unicode-xid"
 version = "0.2.6"