Announce tick-inferred routines, opt-in and rate-limited (#247, #43)

The digestion tick already runs the pattern detector over all recorded
events (67563ed) and writes a proposed_routines row. What was missing is
the other half of #247: a proposal that nobody is at the mic for reaches
nothing but the /routines page, so a pattern noticed at 03:00 is only
seen if he goes looking.

This wires the tick's proposals into the existing care-delivery path
rather than a second channel: sev1 nudge, loop.Gate, dispatcher, same
routing table as an accepted routine. Restraints, since a feature that
speaks unprompted is the easiest way to turn Maven into a nag:

  - off unless configured — the new pattern_proposals block, absent by
    default, and deploy/mavend.json ships notify: false;
  - at most one announcement per tick however many patterns surfaced;
  - at most one per cooldown (24h default) across all pairs;
  - sev1, so quiet hours, away and snooze suppress it;
  - suppressed means dropped, not queued — /routines still has it;
  - once per pair for good, since proposed_routines is
    UNIQUE(action, object) and the row survives dismissal.

The body is pattern.PhraseRoutine's literal Russian, not LLM-worded, so
an inferred routine cannot arrive describing something never observed.

Also raises pattern.MinEvents from 3 to 4 — the interval-quality item on
#43. Two intervals with a ±50% band is a coincidence with a mean, not a
pattern, and now that a scan of all history can announce itself the cost
of a false positive is a permanent dismissal of that pair.

internal/pattern/detector.go

@@ -20,9 +20,19 @@ type ProposedRoutine struct {
 const MaxIntervalRatio = 1.5
 
 // MinEvents is the minimum number of events needed to detect a pattern.
-// With N events, there are N-1 intervals; we need at least 2 intervals
-// before proposing anything.
-const MinEvents = 3
+// With N events there are N-1 intervals, so 4 events means 3 intervals.
+//
+// This used to be 3 (two intervals), which is not a pattern — it is a
+// coincidence with a mean. Two gaps of similar length happen constantly:
+// water the plants on a Sunday, again the next Sunday, once more the Sunday
+// after, and a detector with a ±50% band calls that a weekly routine. The
+// cost of being wrong is asymmetric now that the digestion tick scans all of
+// history on its own schedule and can announce what it finds: a false
+// positive is something the owner has to read and dismiss, and a dismissal
+// is permanent, so one bad guess burns that action+object pair forever.
+// Three intervals is the cheapest bar that makes a run distinguishable from
+// a repeat. False negatives cost one more observation and nothing else.
+const MinEvents = 4
 
 // Detect checks whether a sequence of events for the same action+object
 // forms a stable recurring pattern. Returns a ProposedRoutine when: