feat: add PostgreSQL introspection support (#14)

* feat(backend): add PostgreSQL introspection support with AbstractIntrospector

Implement introspection for PostgreSQL backend using pg_catalog system tables:

- Add PostgreSQLIntrospector using pg_catalog for metadata queries
- Add IntrospectorBackendMixin to PostgresBackend
- Add complete test suite covering all introspection features

Key features:
- Queries pg_class, pg_attribute, pg_index, pg_constraint, pg_trigger
- Default schema is 'public'
- Supports tables, columns, indexes, foreign keys, views, triggers

This aligns PostgreSQL with the new introspection architecture.

* refactor(backend): separate sync/async introspector implementations

Split PostgreSQLIntrospector into:
- PostgreSQLIntrospectorMixin: shared non-I/O logic
- SyncPostgreSQLIntrospector: synchronous introspector
- AsyncPostgreSQLIntrospector: asynchronous introspector

This aligns with the core package refactoring and follows the
project's sync/async parity principle where sync and async are
separate and cannot coexist in the same class.

* feat(backend): add introspect subcommand to PostgreSQL CLI

Add a new 'introspect' subcommand to the PostgreSQL backend CLI for
database introspection operations:
- Tables, views, columns listing
- Indexes, foreign keys, triggers info
- Database information

Also includes code style fixes across all files:
- Format code with ruff format
- Fix line length violations (E501)
- Remove unused imports (F401)
- Add 'from None' to exception chaining (B904)

* refactor(cli): use explicit subcommand mode and fix Chinese to English

- Use parent parser pattern for shared connection parameters
- Separate query and introspect subcommands explicitly
- Fix Chinese comments to English in design notes
- Rename args.query to args.sql for consistency

* refactor(backend): delegate PostgreSQL introspection SQL generation to dialect

Remove _build_*_sql() methods from PostgreSQLIntrospectorMixin, relying
on the base class implementation that delegates to Expression.to_sql().

PostgresIntrospectionCapabilityMixin.format_*_query() methods already
exist and now handle all SQL generation.

Changes:
- Removed 8 _build_*_sql() methods from introspector.py
- Updated _parse_database_info() to use 'version' and 'collation' columns
- Updated _parse_triggers() to use 'timing' column from Dialect
- Removed get_database_info() override (Dialect returns version in single query)
- Fixed psycopg3 % escaping: 'pg_%' -> 'pg_%%'

No functional changes - SQL output and API remain identical.

* fix(introspection): correct PostgreSQL trigger timing detection

The tgtype bitmask was incorrectly interpreted for trigger timing:
- Bit 2 (4) is INSERT event, not AFTER timing
- Bit 3 (8) is DELETE event, not INSTEAD OF timing
- AFTER timing has neither bit 1 nor bit 6 set (default case)
- INSTEAD OF uses bit 6 (64), not bit 3

Corrected SQL CASE expression:
- INSTEAD OF: bit 6 (64) set
- BEFORE: bit 1 (2) set
- AFTER: neither (default)

Added comprehensive test cases for:
- AFTER INSERT trigger
- AFTER UPDATE trigger
- AFTER DELETE trigger
- INSTEAD OF trigger on view

All 24 trigger introspection tests now pass.

* ci: update rhosocial-activerecord to git+https with release/v1.0.0.dev20

Change installation source from PyPI version 1.0.0.dev19 to git repository
branch release/v1.0.0.dev20 for CI testing.

* fix(introspection): handle PostgreSQL 14+ reltuples returning -1 for unanalyzed tables

PostgreSQL 14+ returns -1 for pg_class.reltuples when a table has never
been vacuumed or analyzed. This commit converts -1 to None to maintain
cross-version consistency.

- Converts reltuples=-1 to row_count=None in _parse_tables()
- Ensures consistent behavior across PostgreSQL 9-18
- Fixes test_table_row_count failure on PostgreSQL 14+

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

* chore: add changelog fragment for PR #14

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: vistart

.github/workflows/test.yml

@@ -65,9 +65,9 @@ jobs:
           pip install -r requirements.txt
           pip install -r requirements-dev.txt
 
-      - name: Install activerecord and testsuite from PyPI
+      - name: Install activerecord and testsuite from git
         run: |
-          pip install rhosocial-activerecord==1.0.0.dev19
+          pip install git+https://github.com/rhosocial/python-activerecord.git@release/v1.0.0.dev20
           pip install rhosocial-activerecord-testsuite==1.0.0.dev7
 
       - name: Create postgres scenario config

changelog.d/14.added.md

@@ -0,0 +1 @@
+Added PostgreSQL introspection support with AbstractIntrospector for database schema discovery, including tables, columns, indexes, foreign keys, views, and triggers.

docs/en_US/postgres_specific_features/README.md

@@ -9,6 +9,7 @@ PostgreSQL offers many advanced features beyond standard SQL. This section cover
 - **[Advanced Indexing](./indexing.md)**: GIN, GiST, BRIN indexes
 - **[RETURNING Clause](./returning.md)**: Get data back from DML operations
 - **[Protocol Support Matrix](./protocol_support.md)**: Comprehensive protocol support and version compatibility
+- **[Database Introspection](./introspection.md)**: Query metadata using pg_catalog
 
 ## Feature Highlights