feat: add task blocker dependency graph + actionable now view

- New task_blockers join table (many-to-many, cross-project)
- Cycle detection prevents circular dependencies
- GET /api/tasks/{id}/blockers and /blocking endpoints
- POST/DELETE /api/tasks/{id}/blockers/{blocker_id}
- GET /api/actionable — all non-done tasks with no incomplete blockers
- BlockerPanel.jsx — search & manage blockers per task (via task menu)
- ActionableView.jsx — "what can I do right now?" dashboard grouped by project
- "Now" button in nav header routes to actionable view
- migrate_add_blockers.py migration script for existing databases

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

backend/migrate_add_blockers.py

@@ -0,0 +1,40 @@
+"""
+Migration script to add the task_blockers association table.
+Run this once if you have an existing database.
+
+Usage (from inside the backend container or with the venv active):
+    python migrate_add_blockers.py
+"""
+import sqlite3
+import os
+
+db_path = os.path.join(os.path.dirname(__file__), 'bit.db')
+
+if not os.path.exists(db_path):
+    print(f"Database not found at {db_path}")
+    print("No migration needed — new database will be created with the correct schema.")
+    exit(0)
+
+conn = sqlite3.connect(db_path)
+cursor = conn.cursor()
+
+try:
+    # Check if the table already exists
+    cursor.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='task_blockers'")
+    if cursor.fetchone():
+        print("Table 'task_blockers' already exists. Migration not needed.")
+    else:
+        cursor.execute("""
+            CREATE TABLE task_blockers (
+                task_id       INTEGER NOT NULL REFERENCES tasks(id) ON DELETE CASCADE,
+                blocked_by_id INTEGER NOT NULL REFERENCES tasks(id) ON DELETE CASCADE,
+                PRIMARY KEY (task_id, blocked_by_id)
+            )
+        """)
+        conn.commit()
+        print("Successfully created 'task_blockers' table.")
+except Exception as e:
+    print(f"Error during migration: {e}")
+    conn.rollback()
+finally:
+    conn.close()