feat: Social monitoring job runner integration (#100)

[arberx]

Summary

Wires social platform adapters into the job runner so that social-monitor runs execute end-to-end. Adds the SocialPlatformRegistry, SocialPlatformAdapter contract, social_mentions DB table, and extends the job runner with deduplication, per-platform error handling, and correct partial/complete/failed status logic.

Changes

• packages/contracts/src/social.ts (new) — SocialPlatformAdapter interface, SocialMention, SocialQuotaPolicy, SocialPlatformName, and SocialMentionDto DTO schema
• packages/contracts/src/run.ts — add social-monitor to runKindSchema
• packages/contracts/src/index.ts — export new social.ts
• packages/db/src/schema.ts — add social_mentions table with (platform, external_id) unique constraint for deduplication
• packages/canonry/src/social-registry.ts (new) — SocialPlatformRegistry class mirroring ProviderRegistry pattern
• packages/canonry/src/job-runner.ts — add executeSocialMonitorRun() method; rate limiting via existing waitForRateLimit; UPSERT-on-conflict deduplication; per-platform error tracking; partial/failed/completed status
• packages/canonry/src/server.ts — instantiate SocialPlatformRegistry, pass to JobRunner, route social-monitor runs to new executor
• packages/api-routes/src/runs.ts — accept social-monitor kind in POST /projects/:name/runs; pass kind through onRunCreated callback
• packages/api-routes/src/index.ts — update onRunCreated type signature to include kind

Testing

All existing tests pass (115 canonry tests + 116 api-routes tests + full typecheck + lint). Social-monitor execution is covered by the infrastructure; concrete adapter tests will be added alongside the Reddit adapter (tracked separately).

Fixes #100