Phase 2: Gmail IMAP fetch (IPv4), FR-2 source address, real mail in dashboard (AUD-67)

- internal/mailfetch: stdlib-only read-only IMAP client (LOGIN, SELECT,
  UID SEARCH, UID FETCH) that always dials tcp4; IPv6 to Gmail is broken
  from .13 and caused a socket EOF.
- MIME parsing to text, RFC2047 header decoding, FR-2 source address:
  X-Pm-Original-To -> To -> Received 'for <addr>' -> Return-Path, never
  Delivered-To. The matched header is recorded for audit.
- store: Message upsert/list/count + LogAction; schema migration v2 adds
  imap_uid and imap_uidvalidity.
- server: dashboard and /partials/inbox render real mail grouped by
  category with a recommended action; POST /actions/fetch runs a scan
  (CSRF-checked); nav counts; 'Check mail' button.
- config: GMAIL_USER, GMAIL_APP_PASSWORD, IMAP host/port/lookback/max.
- cmd: -fetch runs one scan and exits (n8n/CLI entry point, FR-1).
- compose/.env.example expose the Gmail + IMAP settings.
- tests: FR-2 precedence, MIME bodies, IMAP literal/parser, store
  upsert/migration, inbox rendering.

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

internal/store/schema.sql

@@ -24,6 +24,10 @@ CREATE TABLE IF NOT EXISTS messages (
     body_text       TEXT,
     source_address  TEXT,
     date_sent       TEXT,
+    -- IMAP bookkeeping (AGENTS.md §9): the mailbox UID and the UIDVALIDITY it
+    -- belongs to. UIDs are only meaningful while UIDVALIDITY is unchanged.
+    imap_uid            INTEGER,
+    imap_uidvalidity    INTEGER,
     received_at     TEXT    NOT NULL DEFAULT (datetime('now')),
     created_at      TEXT    NOT NULL DEFAULT (datetime('now'))
 );