Add thread-safe connection handling and transaction support

[ncoop57]

Refactors fastsql to be safe for concurrent use (sync web frameworks, threaded workers):

• Connection management: Remove persistent db.conn; use connection pool with per-operation checkout. Database.conn(write=) context manager handles connection lifecycle.
• Transaction support: Add db.tx() for explicit transaction scopes with automatic commit/rollback. Nested calls use SAVEPOINTs.
• Thread-safe state: DBTable.xtra_id and result now use contextvars so each thread/task gets isolated state.
• Thread-safe table cache: Database._tables protected by lock.
• Simplified execute: db.execute() now buffers and returns results directly (list of mappings or rowcount).
• Pass engine_kws through database() for pool configuration (pool_size, max_overflow, etc.)

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