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-entities/src/lib.rs

@@ -104,6 +104,25 @@ pub struct ProjectSummary {
     pub last_activity: Option<DateTime<Utc>>,
 }
 
+/// Per-language daily commit count for the language stream graph.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct LanguageDailyCount {
+    pub date: chrono::NaiveDate,
+    pub language: String,
+    pub color: Option<String>,
+    pub commits: i64,
+}
+
+/// Per-repo language breakdown from the forge.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct RepoLanguage {
+    pub source: Source,
+    pub repo: String,
+    pub language: String,
+    pub bytes: i64,
+    pub color: Option<String>,
+}
+
 // ---------------------------------------------------------------------
 // Presentation shape — what `GET /v1/events` actually returns.
 // The API reshapes raw payloads into these so the frontend stays dumb.