feat: add database module with connection factory and transactions

Implements Step 1.2:
- constants.py: END_OF_TIME_UTC = 32503680000000 (year 3000 sentinel)
- db.py: get_db() with pragmas (WAL, synchronous=FULL, foreign_keys, busy_timeout)
- db.py: transaction() context manager with BEGIN IMMEDIATE

Includes 12 TDD tests for pragmas, commit/rollback, and concurrent writes.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

PLAN.md

@@ -16,11 +16,11 @@ Check off items as completed. Each phase builds on the previous.
 - [x] **Commit checkpoint** (61f704c)
 
 ### Step 1.2: Database Connection & Pragmas
-- [ ] Create `db.py` with connection factory
-- [ ] Set pragmas: WAL, synchronous=FULL, foreign_keys=ON, busy_timeout=5000
-- [ ] Create transaction context manager (BEGIN IMMEDIATE)
-- [ ] Create `constants.py` with END_OF_TIME_UTC
-- [ ] Write tests for pragmas and transactions
+- [x] Create `db.py` with connection factory
+- [x] Set pragmas: WAL, synchronous=FULL, foreign_keys=ON, busy_timeout=5000
+- [x] Create transaction context manager (BEGIN IMMEDIATE)
+- [x] Create `constants.py` with END_OF_TIME_UTC
+- [x] Write tests for pragmas and transactions
 - [ ] **Commit checkpoint**
 
 ### Step 1.3: Migration Framework