Fix gig account filtering and content link parsing

[SummusStuprator]

Three small correctness fixes, each in its own commit:

• Render mentions using the same start/whitespace/parenthesis boundary used for notifications. alice@example.com currently becomes a profile link to example; it now remains text. Real mentions preserve their surrounding text and displayed case.
• Require the whole GitHub PR number segment. /pull/12abc and /pull/12.5 currently pass link validation even though the PR-coordinate parser rejects them. Genuine PR URLs, subpaths and search/list URLs remain supported.
• Filter gigs through the selected poster relation alias with an inner join when account_type is supplied. Without the inner join, filtering the embedded profile does not exclude nonmatching gigs. Unfiltered searches retain their left join.

Validation:

• New mention/link regressions failed before the fixes; their final suites and the gig route suite pass (40 tests).
• Full configured suite: 220 files, 2,115 tests pass.
• ESLint: 0 errors, 40 warnings. Standalone tsc --noEmit: passes.
• Draft because the full production build is not green locally. The existing test-only node_modules junction initially blocked Turbopack. After replacing it with a regular installation, the build compiled successfully in 11.9 seconds, but its TypeScript worker exhausted the build script's configured 2 GB heap. No build configuration changed in this PR.
• The Git Bash commit hook could not find pnpm. Its lint/type/test/build checks were run directly; commits were recorded with the hook disabled for those individual commands after recording the results above. No persistent hook configuration was changed.

Prepared by an AI coding assistant with the account owner's authorization. No live database/customer data used, no production changes. The earlier account-filter patch is included as an open-source contribution; this PR is not an invoice or assertion of an agreed bounty.