feat: add external database support for plan command

[tianzhou]

Allow users to optionally provide an external PostgreSQL database for plan generation instead of always using embedded postgres. This addresses limitations in issues #121 and #122.

Key changes:

• Add DesiredStateProvider interface to abstract embedded/external DB
• Implement ExternalDatabase for user-provided databases
• Add --plan-host, --plan-port, --plan-db, --plan-user, --plan-password flags
• Support PGSCHEMA_PLAN_* environment variables
• Create temporary schemas with nanosecond timestamps for isolation
• Validate major version compatibility between plan and target DB
• Maintain backwards compatibility (embedded postgres remains default)

The external database creates temporary schemas (pgschema_plan_) and cleans them up after plan generation (best effort).

🤖 Generated with Claude Code

[Copilot]

Pull Request Overview

This PR introduces support for using an external PostgreSQL database as the plan database instead of always using an embedded PostgreSQL instance. This allows users to optionally specify an external database via --plan-host and related flags for generating migration plans.

• Introduces a DesiredStateProvider interface to abstract embedded vs external database functionality
• Adds ExternalDatabase implementation that uses temporary schemas with timestamps for isolation
• Updates the plan and apply commands to support external database configuration via flags and environment variables

Reviewed Changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/postgres/desired_state.go	Defines the DesiredStateProvider interface to abstract desired state database implementations
internal/postgres/external.go	Implements ExternalDatabase for connecting to external PostgreSQL databases with version checking and temporary schema management
internal/postgres/embedded.go	Adds GetSchemaName() method to implement DesiredStateProvider interface
cmd/plan/plan.go	Updates to use DesiredStateProvider interface and adds plan-* flags for external database configuration
cmd/apply/apply.go	Updates to use DesiredStateProvider interface instead of concrete EmbeddedPostgres type
cmd/util/env.go	Adds helper functions for plan database environment variable handling and validation
cmd/plan/external_db_integration_test.go	Integration tests for external database functionality
go.mod, go.sum	Updates dependencies (moves testify from indirect to direct)

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