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-core/src/lib.rs

@@ -6,7 +6,7 @@ pub use sources::{EventSource, PollerState, PollerStateStore, SourceError, run_p
 
 use async_trait::async_trait;
 use chrono::NaiveDate;
-use moments_entities::{DailyCount, Event, EventQuery, ProjectSummary, SourceSummary};
+use moments_entities::{DailyCount, Event, EventQuery, LanguageDailyCount, ProjectSummary, RepoLanguage, SourceSummary};
 
 #[derive(Debug, thiserror::Error)]
 pub enum StoreError {
@@ -21,10 +21,13 @@ pub trait EventReader: Send + Sync {
     async fn source_summaries(&self, include_private: bool) -> Result<Vec<SourceSummary>, StoreError>;
     async fn list_projects(&self) -> Result<Vec<ProjectSummary>, StoreError>;
     async fn daily_counts(&self, from: NaiveDate, to: NaiveDate) -> Result<Vec<DailyCount>, StoreError>;
+    async fn language_daily_counts(&self, from: NaiveDate, to: NaiveDate) -> Result<Vec<LanguageDailyCount>, StoreError>;
+    async fn repo_languages(&self) -> Result<Vec<RepoLanguage>, StoreError>;
 }
 
 /// Write-side port consumed by `moments-worker`. Idempotent upserts on `id`.
 #[async_trait]
 pub trait EventWriter: Send + Sync {
     async fn upsert_events(&self, events: &[Event]) -> Result<usize, StoreError>;
+    async fn upsert_repo_languages(&self, languages: &[RepoLanguage]) -> Result<usize, StoreError>;
 }