#4: implement F2 sub-project 1 - IMAP core (auth, LIST, SELECT, FETCH, STORE)

Adds a real :143 IMAP listener to the Go protocols service: a mailbox
owner authenticates over IMAP (Argon2id against mailboxes.password_hash,
enumeration-resistant per PLAN.md section 14.6), lists folders, selects
INBOX, fetches messages F1's SMTP path already persisted - including real
ENVELOPE/BODY[] built from the actual stored MIME bytes via go-imap/v2's
own ExtractEnvelope/ExtractBodySection helpers, not reconstructed from
denormalized columns - and sets/reads flags via STORE.

- ImapSessionUseCase orchestrates auth, folder listing/selection, message
  querying, and flag mutation against new Postgres repositories (auth,
  folders, message query, flags) and a local-disk blob reader.
- A github.com/emersion/go-imap/v2 (pinned v2.0.0-beta.8) Backend/Session
  implements 7 real methods (Login, List, Select, Unselect, Status, Fetch,
  Store) plus a correct no-op Poll; the other 10 of the interface's 18
  methods (Create/Delete/Rename/Subscribe/Unsubscribe/Append/Idle/Expunge/
  Search/Copy) are explicit stubs returning a clear "not yet implemented"
  IMAP error - SEARCH/EXPUNGE/COPY/MOVE, IDLE/CONDSTORE, mailbox
  management, and APPEND are separate follow-up sub-projects per the
  design doc's agreed ordering with the user.

Verified end-to-end with a real go-imap/v2 imapclient (embedded Postgres,
no Docker needed) driving Login -> Select -> Fetch -> Store -> List
against a message seeded through F1's real SMTP use case, proving true
SMTP-write/IMAP-read interop, not just isolated unit tests.

Built via 8 sequential tasks under subagent-driven development, each
independently reviewed, plus a final whole-branch review. Two real bugs
were found and fixed before merge, both exactly the kind unit tests with
fakes cannot catch:
- Store's UID-set resolution silently no-op'd on dynamic ranges like
  "UID STORE 1:*" (the common "mark all as read" operation) after an
  earlier draft bypassed the folder's real message list; restored to
  resolve against real messages via the same pattern Fetch already used
  correctly, with tests proving both dynamic ranges and non-UID STORE.
- go-imap/v2's server calls Session.Poll() implicitly before the status
  response of nearly every authenticated command (FETCH/STORE/SEARCH/
  APPEND/COPY), not just IDLE as assumed - a hard-stubbed Poll broke every
  real command through this path; fixed to a correct no-op.
- List had zero test coverage despite being committed scope; added unit
  tests for wildcard/delimiter matching plus a real E2E LIST assertion.

Known follow-up, not blocking: a pre-existing test-fixture hygiene gap
(hardcoded content_sha256 blob hashes with no cleanup, shared across
several Postgres integration test files including one new to this
branch) causes local-dev test collisions on repeated runs against a
persistent database - invisible to CI (fresh Postgres per run) but worth
a cleanup pass.

services/protocols/internal/infrastructure/postgres/flag_repository.go

@@ -0,0 +1,72 @@
+package postgres
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"lambdamail/protocols/internal/application/port"
+)
+
+// FlagRepository implements port.FlagRepository against Postgres.
+type FlagRepository struct {
+	pool *pgxpool.Pool
+}
+
+func NewFlagRepository(pool *pgxpool.Pool) *FlagRepository {
+	return &FlagRepository{pool: pool}
+}
+
+func (r *FlagRepository) SetFlags(ctx context.Context, folderID string, uid uint32, op port.FlagOp, flags []string) error {
+	tx, err := r.pool.Begin(ctx)
+	if err != nil {
+		return fmt.Errorf("begin transaction: %w", err)
+	}
+	defer tx.Rollback(ctx)
+
+	var messageID string
+	var receivedAt any
+	if err := tx.QueryRow(ctx, `
+		SELECT id, received_at FROM email_messages WHERE folder_id = $1 AND uid = $2
+	`, folderID, uid).Scan(&messageID, &receivedAt); err != nil {
+		return fmt.Errorf("find message folder=%s uid=%d: %w", folderID, uid, err)
+	}
+
+	switch op {
+	case port.FlagOpAdd:
+		for _, flag := range flags {
+			if _, err := tx.Exec(ctx, `
+				INSERT INTO message_flags (message_id, received_at, flag) VALUES ($1, $2, $3)
+				ON CONFLICT DO NOTHING
+			`, messageID, receivedAt, flag); err != nil {
+				return fmt.Errorf("add flag %q: %w", flag, err)
+			}
+		}
+	case port.FlagOpDel:
+		for _, flag := range flags {
+			if _, err := tx.Exec(ctx, `
+				DELETE FROM message_flags WHERE message_id = $1 AND received_at = $2 AND flag = $3
+			`, messageID, receivedAt, flag); err != nil {
+				return fmt.Errorf("delete flag %q: %w", flag, err)
+			}
+		}
+	case port.FlagOpSet:
+		if _, err := tx.Exec(ctx, `DELETE FROM message_flags WHERE message_id = $1 AND received_at = $2`, messageID, receivedAt); err != nil {
+			return fmt.Errorf("clear flags before set: %w", err)
+		}
+		for _, flag := range flags {
+			if _, err := tx.Exec(ctx, `
+				INSERT INTO message_flags (message_id, received_at, flag) VALUES ($1, $2, $3)
+				ON CONFLICT DO NOTHING
+			`, messageID, receivedAt, flag); err != nil {
+				return fmt.Errorf("set flag %q: %w", flag, err)
+			}
+		}
+	}
+
+	if err := tx.Commit(ctx); err != nil {
+		return fmt.Errorf("commit: %w", err)
+	}
+	return nil
+}