crawler: --skip-ext + --max-source-mb gates for batch-50 expansion

Two CLI gates needed before scaling Pro batch beyond top-5:

--skip-ext mp4,qt,mov  (attachment filter)
  Skips video extensions in attachment download. Phase 1 measurements
  showed mp4+qt occupy ~54% of attachment storage. Entry still recorded
  in metadata.json with skipped:ext:<token> so we can re-fetch later if
  the policy changes. Honors both server-declared `ext` and filename
  suffix, case-insensitively.

--max-source-mb N  (Pro source size cap)
  Trips inside the chain replay loop on encrypted-blob total. On trip:
  raise ProjectOversizeError, wipe partial source/, append a row to
  data/state/oshwhub_pro_oversize.jsonl. Lets us shortlist 50+ Pro
  projects without one X86-board-class outlier (~500 MB) blowing the
  LFS budget. Std and Pro 2.x legacy are not capped (both <2 MB in
  sample).

Verified:
  - cap=0 trips on first blob (1.2 MB), source/ wiped, state recorded
  - cap=100 runs full ESP-VoCat (7.5 MB plain, 278 docs)
  - skip-ext microtest: 8/8 cases (case-insensitive, declared/suffix
    fallback, empty-token edge cases)

Plan + frozen candidate list for the next 50 projects:
  - docs/plans/oshwhub_batch50.md
  - data/state/oshwhub_batch50_candidates.jsonl (gitignore exception added)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

data/raw/oshwhub/ba64bd6f1c9c467ba3b674a54943557d/source/manifest.json

@@ -2,7 +2,7 @@
   "project_uuid": "ba64bd6f1c9c467ba3b674a54943557d",
   "branch_uuid": "ef5f58bd0f1245b0a808c07e541a1b5c",
   "head_uuid": "764dd8b722a44914a915493277e204c9",
-  "fetched_at": "2026-04-28T16:06:55.434479+00:00",
+  "fetched_at": "2026-04-28T16:23:06.697278+00:00",
   "editor_version": "3.2.91",
   "chain_length": 12,
   "blob_bytes_total": 1195716,