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/filter/filter.go

@@ -0,0 +1,52 @@
+// Package filter provides message filtering configuration.
+// It is used to restrict which messages are processed by the answer workers.
+//
+// NOTE: IMAP Search Limitations
+// IMAP SEARCH (RFC 3501 §6.4.4) uses case-insensitive substring matching for
+// text fields.
+// A filter like From: "alice@example.com" would match:
+//   - alice@example.com
+//   - malice@example.com
+//   - alice@example.com.evil.org
+//
+// For this reason, server-side IMAP filters should be considered
+// performance optimizations only, not security controls. Use the
+// [Filters.Senders] allowlist for exact sender matching.
+//
+// [RFC 3501 §6.4.4]:
+// https://datatracker.ietf.org/doc/html/rfc3501#section-6.4.4
+package filter
+
+import "strings"
+
+// Filters holds message filtering configuration for IMAP searches
+// and sender verification.
+type Filters struct {
+	// Body contains keywords that must appear in the message body.
+	Body []string `yaml:"body"`
+	// From filters messages by the From header field.
+	From string `yaml:"from"`
+	// Senders is an allowlist of email addresses permitted to receive
+	// replies. If empty, all senders are allowed.
+	Senders []string `yaml:"allowed_senders"`
+	// Subject filters messages by the Subject header field.
+	Subject string `yaml:"subject"`
+	// To filters messages by the To header field.
+	To string `yaml:"to"`
+}
+
+// MatchSender checks if the sender is in the allowed list.
+// Returns true if the allowlist is empty (allow all) or if the sender matches.
+// Comparison is case-insensitive and ignores leading/trailing whitespace.
+func (f *Filters) MatchSender(sender string) bool {
+	if len(f.Senders) == 0 {
+		return true
+	}
+	sender = strings.ToLower(strings.TrimSpace(sender))
+	for _, allowed := range f.Senders {
+		if strings.ToLower(strings.TrimSpace(allowed)) == sender {
+			return true
+		}
+	}
+	return false
+}