Add /tldr: a private overview of what you missed in a channel

/tldr [since] summarizes the messages in the channel it is run in that the
caller has not been summarized yet, using a small local model on Ollama, and
replies ephemerally. The answer streams into the reply as it is written.

- Per-user, per-channel cursor (tldr_cursors) so each run picks up where the
  last left off; `since:` (e.g. 6h, 2d, max 7d) peeks without moving it.
  First use covers the last 24h, and a stale cursor never reaches back more
  than 7 days.
- Only summarizes channels both the caller and the bot can read.
- The transcript drops bots, system messages, reactions and filler, turns
  links, GIFs, images and files into short tags ([link: site], [image],
  [file: name], preview titles), marks replies, and writes a date marker
  where the chat paused for 3h or more so the model can keep topics apart.
- The prompt asks for one bullet per conversation, oldest first, with a
  worked example; it was tuned against a synthetic chat and a real export
  with qwen3:4b-instruct-2507 (the default model).
- One model call at a time across the bot, one /tldr per user.
- docker-compose: Ollama service (CPU only, 8g memory cap: 6g killed the
  runner mid-request), model pull on startup.

Needs the tldr_cursors migration (velox-dashboard-api 000016).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

internal/tldr/ids.go

@@ -0,0 +1,55 @@
+package tldr
+
+import (
+	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
+	"time"
+)
+
+const discordEpochMs = 1420070400000
+
+var sinceRe = regexp.MustCompile(`^(\d+)([mhd])$`)
+
+// ParseSince turns "30m", "6h" or "2d" into a duration, clamped to
+// [1m, maxWindow]. time.ParseDuration has no day unit, hence the custom parser.
+func ParseSince(s string) (time.Duration, error) {
+	m := sinceRe.FindStringSubmatch(strings.ToLower(strings.TrimSpace(s)))
+	if m == nil {
+		return 0, fmt.Errorf("couldn't read %q, use something like 30m, 6h or 2d", s)
+	}
+	n, err := strconv.Atoi(m[1])
+	if err != nil || n <= 0 {
+		return 0, fmt.Errorf("couldn't read %q, use something like 30m, 6h or 2d", s)
+	}
+
+	unit := map[string]time.Duration{"m": time.Minute, "h": time.Hour, "d": 24 * time.Hour}[m[2]]
+	if n > int(maxWindow/unit) {
+		return maxWindow, nil
+	}
+	return max(time.Duration(n)*unit, time.Minute), nil
+}
+
+// SnowflakeFromTime returns the smallest Discord snowflake for t, usable as
+// an "after" cursor to mean "messages posted after this moment".
+func SnowflakeFromTime(t time.Time) string {
+	ms := max(t.UnixMilli()-discordEpochMs, 0)
+	return strconv.FormatInt(ms<<22, 10)
+}
+
+// CompareIDs compares two snowflakes numerically (-1, 0, 1).
+func CompareIDs(a, b string) int {
+	ai, aerr := strconv.ParseUint(a, 10, 64)
+	bi, berr := strconv.ParseUint(b, 10, 64)
+	if aerr != nil || berr != nil {
+		return strings.Compare(a, b)
+	}
+	switch {
+	case ai < bi:
+		return -1
+	case ai > bi:
+		return 1
+	}
+	return 0
+}