Phase 4: triage actions (delete, move, mark; bulk select) (AUD-70)

Bulk select in the triage list, with delete/move/mark and a recoverable
delete (FR-6, FR-9).

- migration v4: messages.folder + messages.mailbox; folder/category views
- internal/mailops: app folder -> Gmail label moves over IMAP (IPv4), locate
  by X-GM-MSGID, create labels lazily, move/save/reply/restore, flag marks
- mailfetch: CREATE, UID MOVE (COPY+EXPUNGE fallback), STORE flags, X-GM-MSGID search
- server: POST /actions/bulk with CSRF, UI delete confirmation, second
  confirmation for messages classified Spam (never auto-deleted), per-message
  and summary action-log rows (FR-13)
- inbox: select-all toolbar, checkboxes, folder nav, Restore on non-inbox rows

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/store/schema.sql

@@ -28,11 +28,18 @@ CREATE TABLE IF NOT EXISTS messages (
     -- belongs to. UIDs are only meaningful while UIDVALIDITY is unchanged.
     imap_uid            INTEGER,
     imap_uidvalidity    INTEGER,
+    -- Phase 4 (FR-6/FR-9): the app folder a message is filed under (a low
+    -- cardinality concept used for navigation) and the exact IMAP label it
+    -- currently lives in on Gmail (for example "Delete", "Saved", or
+    -- "Replied-to/hello@where-woof.com"). `folder` defaults to the inbox.
+    folder          TEXT    NOT NULL DEFAULT 'inbox',
+    mailbox         TEXT    NOT NULL DEFAULT 'INBOX',
     received_at     TEXT    NOT NULL DEFAULT (datetime('now')),
     created_at      TEXT    NOT NULL DEFAULT (datetime('now'))
 );
 
 CREATE INDEX IF NOT EXISTS idx_messages_received_at ON messages (received_at);
+CREATE INDEX IF NOT EXISTS idx_messages_folder      ON messages (folder, received_at);
 CREATE INDEX IF NOT EXISTS idx_messages_thread_id   ON messages (thread_id);
 CREATE INDEX IF NOT EXISTS idx_messages_source      ON messages (source_address);