feat: add PostgreSQL database backend support

Add multi-database support allowing selection between SQL Server and
PostgreSQL via database.active config. Changes include dialect-aware
SQL generation, cross-database timestamp functions, PostgreSQL connection
URL builder, and psycopg dependency.

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

README.md

@@ -22,14 +22,24 @@ Edit `config/settings.yaml` to configure your environment:
 
 ```yaml
 database:
+  active: sql_server  # sql_server or postgresql
   sql_server:
     host: your-host
     port: 1433
     database: your-database
     username: your-username
     password: your-password
+  postgresql:
+    host: your-postgres-host
+    port: 5432
+    database: your-database
+    username: your-username
+    password: your-password
 ```
 
+Set `database.active` to choose the database backend. Both backends expect the
+same database name, schemas, and table structure.
+
 ### Local Overrides
 
 For local development, create `config/settings.local.yaml` to override specific values without committing them.