[Feature] API: database selection via query parameter or header

[npozs77]

Description Allow external API callers (service-accounts, MCP server) to target a specific database per request, rather than always using the server's currently active database. ## Use Case An MCP server or automation container authenticated via API key needs to query a specific vocabulary database (e.g., vocabgen-dutch.db) without depending on which database was last selected in the web UI. ## Current Behavior The API always reads from the server's active database (configured via config.yaml db_path or last switched via the web UI database picker). External callers have no way to specify which database to target. ## Proposed Options ### Option A: Query parameter GET /api/words?db=vocabgen-dutch.db ### Option B: Request header X-VocabGen-Database: vocabgen-dutch.db ### Option C: Per-service-account binding in users.yaml yaml service_accounts: - name: mcp-server key_hash: "$2a$..." scope: read-only database: vocabgen-dutch.db ## Acceptance Criteria - WHEN an API request specifies a database, THE server SHALL use that database for the request - WHEN the specified database does not exist, THE server SHALL return 404 JSON - WHEN no database is specified, THE server SHALL use the default (current behavior) - WHEN a service-account has a bound database, THE server SHALL use it regardless of the active UI database ## Notes - This is a follow-up to #101 (API-key auth for service-accounts) - Low priority — default DB is sufficient for most single-DB deployments - Security: restrict to databases in the config directory only (no arbitrary path traversal) ## Target Release v1.6.0 (or later)