Postgres real backend: psycopg adapter, schema, migration

Replace the PostgresDatabase skeleton with a real implementation:
psycopg (v3) first, psycopg2 second, lazy load, missing driver
still raises PostgresDriverMissingError with no silent SQLite
fallback. A sqlite3-style adapter (PgConnection/PgCursor/PgRow)
translates ? placeholders to %s outside string literals, maps
UNIQUE/FK violations, lock errors and connection failures to
explicit Postgres error types, and handles INSERT OR REPLACE,
PRAGMA table_info, AUTOINCREMENT and FTS5 refusal. POSTGRES_SCHEMA
covers all business tables with tasks result columns; server
init_db uses it under CLOUD_DB_BACKEND=postgres, SQLite path
unchanged. migrate_sqlite_to_postgres.py copies rows with
ON CONFLICT DO NOTHING and reseeds identity sequences.
Tests: cloud 239 (235 pass, 4 skipped without a real server),
host 27 pass; real-server tests skip unless CLOUD_TEST_PG_DSN
is set.

cloud/push.py

@@ -33,6 +33,7 @@
 
 import sqlite3
 from cloud.db import connect as _db_connect
+from cloud.db import PgConnection as _PgConnection
 import time
 import uuid
 from contextlib import contextmanager
@@ -70,7 +71,7 @@ MISSING_CREDENTIAL_REASONS = {
 
 
 def _is_connection(db):
-    return isinstance(db, sqlite3.Connection)
+    return isinstance(db, (sqlite3.Connection, _PgConnection))
 
 
 @contextmanager