feat(core): generic HTTP/S file URL capture (Track 1)

- Add crates/archivr-core/src/downloader/http.rs
  - download(url, store_path, timestamp) -> Result<(hash, extension)>
  - Rejects text/html responses with a clear error
  - Derives extension from URL path or Content-Type header
  - Follows redirects (capped at 10), user-agent archivr/0.1
  - 10 unit tests for extension/content-type helpers
- Add Source::Url variant to capture::Source enum
- determine_source: unmatched http/https URLs route to Source::Url
- source_metadata: Source::Url => ("web", "file", "file")
- generate_entry_title: Source::Url arm -> "Downloaded File" fallback
- perform_capture: Source::Url arm calls http::download, uses existing
  temp -> hash_exists -> move_temp_to_raw -> record_media_entry pipeline
- Update test expectations: 3 plain https:// cases now expect Source::Url
- Add reqwest 0.12 (blocking) to workspace and archivr-core deps
- Mark URLs milestone done in docs/README.md
- Update NEXT.md Track 1 status

Cargo.toml

@@ -27,3 +27,4 @@ toml = "0.8.19"
 tower = "0.5.1"
 tower-http = { version = "0.6.2", features = ["fs", "trace"] }
 uuid = { version = "1.18.1", features = ["v4"] }
+reqwest = { version = "0.12", features = ["blocking"] }