Evaluate PRAGMA user_version for schema versioning

Description

GET /version currently exposes schema_version as the lexicographically last migration filename from the schema_migrations table. This requires the table to exist (migrations must have run). SQLite's built-in PRAGMA user_version is a 4-byte integer in the DB header readable on any SQLite file before migrations run.

Acceptance Criteria

• Evaluate whether PRAGMA user_version is needed alongside the filename-based approach
• If yes: update each migration to set PRAGMA user_version to its sequence number; expose both in GET /version
• If no: document in ARCHITECTURE.md why the filename approach is sufficient
• Decision recorded as an ADR in docs/ARCHITECTURE.md

Additional information

The key question is whether any supported use case (e.g. import compatibility checks, tooling that inspects a .db file before attaching it to a running Snackbox instance) requires reading the schema version before migrations have run. If not, the filename approach is sufficient.