feat(store): SQLite with embedded migrations (forgejo-mcp-broker-9jh)

Implements internal/store on top of modernc.org/sqlite (pure-Go, no CGO).
Open applies any pending migrations, Close releases the handle, Ping
underpins /healthz.

Migration design:
- Files embedded via embed.FS under migrations/NNNN_name.sql
- schema_migrations table tracks applied versions; re-open is a no-op
- Each migration runs in its own transaction: no partial commits
- loadMigrations takes an fs.FS so tests can inject synthetic migration
  sets to exercise rollback and conflict paths

Connection pragmas (set via DSN so they apply to every pooled conn):
- journal_mode=WAL — better reader/writer concurrency
- foreign_keys=ON — off by default in SQLite, we always want them
- busy_timeout=5000 — absorb brief contention without surfacing SQLITE_BUSY
- synchronous=NORMAL — standard WAL pairing

Phase 1 schema (0001_initial.sql) is minimal: a broker_meta table with a
schema_version row. Real OAuth tables ship in phase 2.

Tests: 90.1% coverage across public API and internal migration runner,
including bad SQL rollback, PK-conflict record-step failure, and scan
errors on malformed schema_migrations rows.

Closes forgejo-mcp-broker-9jh.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/store/migrations/0001_initial.sql

@@ -0,0 +1,11 @@
+-- 0001_initial.sql
+-- Bootstraps the broker metadata table. Real OAuth schema (clients,
+-- auth_codes, access_tokens, refresh_tokens) lands in phase 2.
+
+CREATE TABLE broker_meta (
+    key   TEXT PRIMARY KEY,
+    value TEXT NOT NULL
+);
+
+INSERT INTO broker_meta (key, value) VALUES
+    ('schema_version', '1');