Add intake worker

Subsystem to monitor IMAP mailbox for new messages.

Introduces:
- intake: worker that uses IDLE or polling to detect new emails.
- imap: client wrapper for connection management and IMAP commands.
- filter: logic for IMAP search and sender allow-list.
- tracker: concurrency control to prevent processing the same UID twice.
- backoff: for handling connection retries with jitter.

internal/tracker/tracker.go

@@ -0,0 +1,43 @@
+// Package tracker prevents duplicate processing of IMAP messages by
+// maintaining a transient set of active, in-flight UIDs.
+package tracker
+
+import (
+	"sync"
+
+	"github.com/emersion/go-imap/v2"
+)
+
+// Tracker ensures unique concurrent processing of messages.
+// This prevents race conditions where a message might be fetched
+// again before its initial processing completes.
+type Tracker struct {
+	mu  sync.Mutex
+	ids map[imap.UID]struct{}
+}
+
+// New returns a ready-to-use Tracker.
+func New() *Tracker {
+	return &Tracker{
+		ids: make(map[imap.UID]struct{}),
+	}
+}
+
+// TryAcquire attempts to claim exclusive processing rights for a UID.
+// It returns true only if the UID is not currently being tracked.
+func (t *Tracker) TryAcquire(uid imap.UID) bool {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if _, exists := t.ids[uid]; exists {
+		return false
+	}
+	t.ids[uid] = struct{}{}
+	return true
+}
+
+// Release relinquishes processing rights for a UID, allowing it to be acquired again.
+func (t *Tracker) Release(uid imap.UID) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	delete(t.ids, uid)
+}