feat: add Alembic database migration system

Implements Alembic for managing token storage database schema versions.
Migrations run automatically on startup with full backward compatibility.

**Changes:**
- Add Alembic dependency (1.14.0+) and SQLAlchemy (auto-installed)
- Create migration infrastructure in alembic/ directory
- Add initial migration (001) capturing current schema
- Modify RefreshTokenStorage.initialize() to run migrations via anyio
- Add CLI commands: db upgrade, current, history, downgrade, migrate
- Add comprehensive migration documentation

**Backward Compatibility:**
- Pre-Alembic databases automatically stamped with revision 001
- No schema changes for existing databases
- Automatic upgrade on first startup after update

**Migration Strategy:**
Three scenarios handled:
1. New database → Run migrations from scratch
2. Pre-Alembic database → Stamp with 001 (no changes)
3. Alembic-managed → Upgrade to latest

**Architecture:**
- Uses anyio.to_thread.run_sync() for structured concurrency
- Alembic env.py runs with anyio.run() in worker thread
- SQLite-friendly migration patterns documented
- No ThreadPoolExecutor needed (anyio handles it)

**CLI Usage:**
```bash
nextcloud-mcp-server db upgrade    # Upgrade to latest
nextcloud-mcp-server db current    # Show version
nextcloud-mcp-server db history    # View changelog
nextcloud-mcp-server db downgrade  # Rollback (with confirmation)
nextcloud-mcp-server db migrate "description"  # Create migration
```

**Testing:**
- All 13 webhook storage tests pass
- New/pre-Alembic database scenarios validated
- anyio integration tested

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

alembic.ini

@@ -0,0 +1,90 @@
+# Alembic configuration file for nextcloud-mcp-server
+
+[alembic]
+# Path to migration scripts
+script_location = alembic
+
+# Template used to generate migration file names
+# Default: %%(rev)s_%%(slug)s
+file_template = %%(year)d%%(month).2d%%(day).2d_%%(hour).2d%%(minute).2d_%%(rev)s_%%(slug)s
+
+# Timezone for migration timestamps
+# Default: utc
+timezone = utc
+
+# Max length of characters to apply to the "slug" field
+# Default: 40
+# truncate_slug_length = 40
+
+# Set to 'true' to run the environment during the 'revision' command
+# Default: false
+# revision_environment = false
+
+# Set to 'true' to allow .pyc and .pyo files without a source .py file
+# Default: false
+# sourceless = false
+
+# Version location specification
+# Supports single or multiple directories
+version_locations = alembic/versions
+
+# Path separator for version locations (required to suppress deprecation warning)
+# Use os (for cross-platform compatibility)
+path_separator = os
+
+# Set to 'true' to search source files recursively in each "version_locations" directory
+# Default: false
+# recursive_version_locations = false
+
+# Output encoding used when revision files are written
+# Default: utf-8
+# output_encoding = utf-8
+
+# Database URL - can be overridden by:
+# 1. Passing -x database_url=... to alembic commands
+# 2. Setting in environment via get_database_url() in env.py
+# Default: sqlite:///app/data/tokens.db
+sqlalchemy.url = sqlite+aiosqlite:////app/data/tokens.db
+
+[post_write_hooks]
+# Post-write hooks allow you to run scripts after generating migration files
+# Example: format migrations with ruff
+# hooks = ruff
+# ruff.type = exec
+# ruff.executable = ruff
+# ruff.options = format REVISION_SCRIPT_FILENAME
+
+# Logging configuration
+[loggers]
+keys = root,sqlalchemy,alembic
+
+[handlers]
+keys = console
+
+[formatters]
+keys = generic
+
+[logger_root]
+level = WARN
+handlers = console
+qualname =
+
+[logger_sqlalchemy]
+level = WARN
+handlers =
+qualname = sqlalchemy.engine
+
+[logger_alembic]
+level = INFO
+handlers =
+qualname = alembic
+
+[handler_console]
+class = StreamHandler
+args = (sys.stderr,)
+level = NOTSET
+formatter = generic
+
+[formatter_generic]
+format = %(levelname)-5.5s [%(name)s] %(message)s
+datefmt = %H:%M:%S