feat: language stream graph on dashboard

Full-stack feature showing programming languages by commit activity
as a stream graph on the dashboard.

Backend:
- migration: repo_languages table (source, repo, language, bytes, color)
- worker: fetch language breakdowns via GitHub GraphQL (batched,
  20 repos/request) and Gitea REST API during poll cycles
- API: GET /v1/languages/daily (daily commit counts per language),
  GET /v1/languages/repos (all stored repo language data)
- fix timezone bug in daily_counts and language_daily_counts: the
  PostgreSQL server timezone (Europe/Sofia, UTC+3) shifted day
  boundaries, miscounting events near midnight. Now uses explicit
  UTC boundaries in generate_series JOINs.
- use per-source CASE for repo name extraction in language query
  to match gitea payload structure (repo.full_name vs repo.name)
- Gitea languages use GitHub colors via COALESCE fallback

Frontend:
- LanguageStreamGraph component: pure SVG stream graph, weekly
  buckets, centered baseline, top 8 languages + Other, GitHub
  canonical language colors, legend with color dots
- DashPage/ProjectPage: fetch repo languages once via new endpoint
  instead of per-repo forge proxy calls (eliminates 200+ GitHub
  API calls and 403 rate limit errors)
- removed fetchLanguages forge proxy wrapper (dead code)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/moments-data/src/hg.rs

@@ -248,6 +248,12 @@ mod tests {
         ) -> Result<usize, moments_core::StoreError> {
             Ok(0)
         }
+        async fn upsert_repo_languages(
+            &self,
+            _languages: &[moments_entities::RepoLanguage],
+        ) -> Result<usize, moments_core::StoreError> {
+            Ok(0)
+        }
     }
     struct NoopState;
     #[async_trait]