feat: per-channel hashcash proof-of-work for PRIVMSG anti-spam

Add per-channel hashcash requirement via MODE +H <bits>. When set,
PRIVMSG to the channel must include a valid hashcash stamp in the
meta.hashcash field bound to the channel name and message body hash.

Server validates stamp format, difficulty, date freshness, channel
binding, body hash binding, and proof-of-work. Spent stamps are
persisted to SQLite with 1-year TTL for replay prevention.

Stamp format: 1:bits:YYMMDD:channel:bodyhash:counter

Changes:
- Schema: add hashcash_bits column to channels, spent_hashcash table
- DB: queries for get/set channel hashcash bits, spent token CRUD
- Hashcash: ChannelValidator, BodyHash, StampHash, MintChannelStamp
- Handlers: validate hashcash on PRIVMSG, MODE +H/-H support
- Pass meta through fanOut chain to store in messages
- Prune spent hashcash tokens in cleanup loop (1-year TTL)
- Client: MintChannelHashcash helper for CLI
- Tests: 12 new channel_test.go + 10 new api_test.go integration tests
- README: document +H mode, stamp format, and usage

internal/db/queries.go

@@ -1305,3 +1305,110 @@ func (database *Database) GetQueueEntryCount(
 
 	return count, nil
 }
+
+// GetChannelHashcashBits returns the hashcash difficulty
+// requirement for a channel. Returns 0 if not set.
+func (database *Database) GetChannelHashcashBits(
+	ctx context.Context,
+	channelID int64,
+) (int, error) {
+	var bits int
+
+	err := database.conn.QueryRowContext(
+		ctx,
+		"SELECT hashcash_bits FROM channels WHERE id = ?",
+		channelID,
+	).Scan(&bits)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"get channel hashcash bits: %w", err,
+		)
+	}
+
+	return bits, nil
+}
+
+// SetChannelHashcashBits sets the hashcash difficulty
+// requirement for a channel. A value of 0 disables the
+// requirement.
+func (database *Database) SetChannelHashcashBits(
+	ctx context.Context,
+	channelID int64,
+	bits int,
+) error {
+	_, err := database.conn.ExecContext(ctx,
+		`UPDATE channels
+		 SET hashcash_bits = ?, updated_at = ?
+		 WHERE id = ?`,
+		bits, time.Now(), channelID)
+	if err != nil {
+		return fmt.Errorf(
+			"set channel hashcash bits: %w", err,
+		)
+	}
+
+	return nil
+}
+
+// RecordSpentHashcash stores a spent hashcash stamp hash
+// for replay prevention.
+func (database *Database) RecordSpentHashcash(
+	ctx context.Context,
+	stampHash string,
+) error {
+	_, err := database.conn.ExecContext(ctx,
+		`INSERT OR IGNORE INTO spent_hashcash
+		 (stamp_hash, created_at)
+		 VALUES (?, ?)`,
+		stampHash, time.Now())
+	if err != nil {
+		return fmt.Errorf(
+			"record spent hashcash: %w", err,
+		)
+	}
+
+	return nil
+}
+
+// IsHashcashSpent checks whether a hashcash stamp hash
+// has already been used.
+func (database *Database) IsHashcashSpent(
+	ctx context.Context,
+	stampHash string,
+) (bool, error) {
+	var count int
+
+	err := database.conn.QueryRowContext(ctx,
+		`SELECT COUNT(*) FROM spent_hashcash
+		 WHERE stamp_hash = ?`,
+		stampHash,
+	).Scan(&count)
+	if err != nil {
+		return false, fmt.Errorf(
+			"check spent hashcash: %w", err,
+		)
+	}
+
+	return count > 0, nil
+}
+
+// PruneSpentHashcash deletes spent hashcash tokens older
+// than the cutoff and returns the number of rows removed.
+func (database *Database) PruneSpentHashcash(
+	ctx context.Context,
+	cutoff time.Time,
+) (int64, error) {
+	res, err := database.conn.ExecContext(ctx,
+		"DELETE FROM spent_hashcash WHERE created_at < ?",
+		cutoff,
+	)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"prune spent hashcash: %w", err,
+		)
+	}
+
+	deleted, _ := res.RowsAffected()
+
+	return deleted, nil
+}