refactor(db): extract message action migration

[Andiveli]

Summary

• Move message-action schema migration behind the internal schema boundary.
• Complete the current schema extraction chain without changing action persistence behavior.

Changes

• Delegate action-column migration from src/db.rs to src/schema.rs.
• Inject deleted-message text narrowly instead of coupling schema code to application state.
• Preserve migration SQL, ordering, idempotence, and runtime projection behavior.
• Add same-target edit/delete migration coverage and document empty persisted edit markers.

Testing

• cargo fmt --check
• Focused schema tests (4 passed)
• Focused message-action tests (7 passed)
• cargo check --lib
• cargo test -- --test-threads=1 (608 passed)
• git diff --check
• Manual testing: N/A; behavior-preserving schema extraction.

Chain context

main
└── PR #261: connection schema setup
    └── PR #264: message schema migrations
        └── 📍 refactor/rust-db-action-schema
            └── next chain: message-store repository extraction

• Depends on: PR refactor(db): extract message schema migrations #264 / refactor/rust-db-message-schema.
• Ends with: schema module ownership of current table creation and migrations.
• Follow-up: begin a separate message-store extraction chain.
• Out of scope: repository behavior fixes, reactions, cursors, cleanup, and worker redesign.
• Review budget: 205 authored lines.
• Rollback: revert 850944d; only src/db.rs and src/schema.rs are affected.

Closes #267