Persist the dialogue session so a restart keeps the conversation

Vikunja #363. The follow-up session was a plain in-memory map, so any
mavend restart dropped the thread. It now mirrors to a small TTL-pruned
sqlite table and is loaded on startup; expired sessions are deleted on
load, not revived. Clarify's pending question is untouched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGeSZxh1DCtRxmFVSYVGvJ

internal/store/dialogue.go

@@ -0,0 +1,74 @@
+package store
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+// DialogueSessionRow — one saved follow-up session. Data is the session
+// encoded by the dialogue package; the store does not look inside it.
+type DialogueSessionRow struct {
+	ID      string
+	Data    []byte
+	Ts      time.Time
+	TTL     time.Duration
+	Expires time.Time
+}
+
+// SaveDialogueSession — write (or replace) the session for one dialogue id.
+// One row per id: a newer turn overwrites the older state.
+func (s *Store) SaveDialogueSession(ctx context.Context, id string, data []byte, ts time.Time, ttl time.Duration) error {
+	expires := ts.Add(ttl)
+	_, err := s.db.ExecContext(ctx, `
+		INSERT INTO dialogue_sessions (id, data, ts, ttl_ms, expires_ts) VALUES (?, ?, ?, ?, ?)
+		ON CONFLICT(id) DO UPDATE SET data       = excluded.data,
+		                              ts         = excluded.ts,
+		                              ttl_ms     = excluded.ttl_ms,
+		                              expires_ts = excluded.expires_ts`,
+		id, data, ts.UnixMilli(), ttl.Milliseconds(), expires.UnixMilli())
+	if err != nil {
+		return fmt.Errorf("save dialogue session: %w", err)
+	}
+	return nil
+}
+
+// DeleteDialogueSession — drop one session (ended, or expired).
+func (s *Store) DeleteDialogueSession(ctx context.Context, id string) error {
+	if _, err := s.db.ExecContext(ctx, `DELETE FROM dialogue_sessions WHERE id = ?`, id); err != nil {
+		return fmt.Errorf("delete dialogue session: %w", err)
+	}
+	return nil
+}
+
+// LoadDialogueSessions — return the sessions still alive at `now` and delete
+// the ones that already ran out. An expired session is dead: it never comes
+// back after a restart.
+func (s *Store) LoadDialogueSessions(ctx context.Context, now time.Time) ([]DialogueSessionRow, error) {
+	if _, err := s.db.ExecContext(ctx,
+		`DELETE FROM dialogue_sessions WHERE expires_ts <= ?`, now.UnixMilli()); err != nil {
+		return nil, fmt.Errorf("prune dialogue sessions: %w", err)
+	}
+	rows, err := s.db.QueryContext(ctx,
+		`SELECT id, data, ts, ttl_ms, expires_ts FROM dialogue_sessions ORDER BY id`)
+	if err != nil {
+		return nil, fmt.Errorf("load dialogue sessions: %w", err)
+	}
+	defer rows.Close()
+	var out []DialogueSessionRow
+	for rows.Next() {
+		var r DialogueSessionRow
+		var tsMilli, ttlMilli, expMilli int64
+		if err := rows.Scan(&r.ID, &r.Data, &tsMilli, &ttlMilli, &expMilli); err != nil {
+			return nil, fmt.Errorf("scan dialogue session: %w", err)
+		}
+		r.Ts = time.UnixMilli(tsMilli).UTC()
+		r.TTL = time.Duration(ttlMilli) * time.Millisecond
+		r.Expires = time.UnixMilli(expMilli).UTC()
+		out = append(out, r)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("load dialogue sessions: %w", err)
+	}
+	return out, nil
+}

internal/store/migrations.go

@@ -74,6 +74,15 @@ ALTER TABLE reminders ADD COLUMN next_fire_ts INTEGER;`, // #2
 	`CREATE INDEX IF NOT EXISTS idx_nudges_snoozed ON nudges (outcome_ts) WHERE outcome = 'snoozed';`, // #8 — SnoozedUntil runs every tick; keep it off a full scan (Vikunja #364)
 	`ALTER TABLE proposed_routines ADD COLUMN accepted_ts INTEGER;
 	ALTER TABLE proposed_routines ADD COLUMN last_fired_ts INTEGER;`, // #9 — accepted routines keep firing (Vikunja #366): the tick loop needs to know when a routine was accepted and when it last nudged
+
+	`CREATE TABLE IF NOT EXISTS dialogue_sessions (
+		id         TEXT PRIMARY KEY,
+		data       BLOB NOT NULL,
+		ts         INTEGER NOT NULL,
+		ttl_ms     INTEGER NOT NULL,
+		expires_ts INTEGER NOT NULL
+	);
+	CREATE INDEX IF NOT EXISTS idx_dialogue_sessions_expires ON dialogue_sessions (expires_ts);`, // #10 — the follow-up session survives a restart (Vikunja #363); small, TTL-pruned table, not a history log
 }
 
 // migrate applies every migration with a number greater than the DB's current