Database Deadlock & Concurrency Race Condition Investigator

You are a Principal Database Administrator and Concurrency Specialist.

Analyze the following database deadlock log and related application transactions:

Deadlock Graph / Error Output:
```sql
ERROR: deadlock detected
Detail: Process 14201 waits for ShareLock on transaction 889102; blocked by process 14205.
Process 14205 waits for ExclusiveLock on tuple (42, 8) of relation "inventory"; blocked by process 14201.
Hint: See server log for query details.
```

Concurrent SQL Queries / Application Code:
```sql
-- Tx A:
UPDATE orders SET status = 'PROCESSING' WHERE id = 101;
UPDATE inventory SET stock = stock - 1 WHERE product_id = 505;

-- Tx B:
UPDATE inventory SET stock = stock + 10 WHERE product_id = 505;
UPDATE orders SET updated_at = NOW() WHERE id = 101;
```

Database Engine: PostgreSQL 16
Isolation Level: Read Committed

Investigate and deliver:
1. **Deadlock Sequence Diagram / Lock Graph**:
   - Reconstruct step-by-step how Transaction A and Transaction B acquired and requested competing locks (RowExclusive, ShareLock, Next-Key Lock, etc.).
   - Identify the exact resource ordering discrepancy.
2. **Root Cause Analysis**:
   - Why did the transactions access tables or rows in differing orders?
   - Did foreign key checks or cascade operations introduce implicit locks?
3. **Architectural Solutions**:
   - Solution 1: Strict consistent locking order in application code.
   - Solution 2: Optimistic locking (`version` column) or `SELECT ... FOR UPDATE SKIP LOCKED`.
   - Solution 3: Query restructuring or batching improvements.
4. **Corrected SQL / Code Implementation**:
   - Provide the refactored transaction logic.