Feature request: add `--migrate-schema` flag to `start` for non-breaking schema changes without resetting indexer state

[saurabhburade]

Problem

Currently, evolving the schema (e.g., adding new fields) risks schema mismatch or requires resetting/reindexing. This makes incremental, backward-compatible schema updates difficult during active indexing.

Goal

Allow additive, non-breaking schema updates while continuing indexing from the existing checkpoint — without resetting indexer state.

Specifically:

• Add new fields to entities.
• Ensure new fields are nullable/optional.
• Continue indexing from the current state.
• Preserve compatibility with already indexed records.

Proposed Solution

Introduce a schema migration flag for the start command:

Expected Behavior

• Detect schema differences between the current DB schema and project schema.
• Automatically apply safe, additive changes (e.g., adding nullable columns).
• Do not reset indexer state or checkpoint.
• Fail with a clear error if changes are breaking (e.g., type changes, non-null additions without defaults, column removals).

Acceptance Criteria

• ✅ Adding nullable fields does not require a reindex.
• ✅ Indexer continues from the existing checkpoint.
• ✅ Old records remain valid (new fields default to NULL).
• ✅ Clear output describing applied migrations.
• ❌ Breaking changes are rejected with actionable messaging.

Rationale

This enables progressive schema evolution without disrupting historical data or requiring expensive reindexing, while maintaining safety guarantees.