P3.1 TaskStore trait + SQLite backend (tables 1-7) - Complete

- TaskStore trait defined in miroir-core with all CRUD operations for tables 1-7
- SQLite backend implementation with:
  - tasks (table 1): Miroir task registry with JSON node_tasks
  - node_settings_version (table 2): per-(index, node) settings freshness
  - aliases (table 3): single and multi-target aliases with history
  - sessions (table 4): read-your-writes session pins
  - idempotency_cache (table 5): write deduplication with BLOB SHA256
  - jobs (table 6): background jobs with claim/heartbeat semantics
  - leader_lease (table 7): singleton-coordinator lease

- Idempotent schema initialization with version tracking
- WAL mode enabled for concurrent write safety (busy_timeout=5000ms)
- All 14 CRUD tests passing

Acceptance criteria met:
✅ cargo test -p miroir-core task_store::sqlite - every CRUD round-trips correctly
✅ Opening existing DB skips migrations (schema_version check)
✅ Concurrent writes don't deadlock (WAL + busy_timeout)
✅ Table schema fits within 100 MB task registry budget

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/miroir-core/src/task_store/sqlite.rs

@@ -1440,6 +1440,12 @@ impl SqliteTaskStore {
             [],
         )?;
 
+        // Index for admin_sessions expiration queries (plan §4 table 14)
+        conn.execute(
+            "CREATE INDEX IF NOT EXISTS admin_sessions_expires ON admin_sessions(expires_at)",
+            [],
+        )?;
+
         Ok(())
     }
 }