Address seed-packet review: 413 mapping, deadline, rollback, dedup

Gadfly findings on #94, the real ones:

- scanSeedPacket extends only the READ deadline; a slow upload + a live
  vision call runs past the server's absolute 30s WriteTimeout and the
  successful response is silently dropped (the #78 failure mode). Extend
  the write deadline too (scanWriteTimeout).
- An oversized upload tripping MaxBytesReader was mapped to 400; it's 413.
  Detect *http.MaxBytesError and report IMAGE_TOO_LARGE.
- Split imagenorm error mapping: ErrTooLarge->413, ErrUnsupported->400,
  genuine read/encode faults (and a failed file.Open)->500, not 400.
- CreateFromPacket discarded the plant it created when the lot then
  failed, contradicting its own doc. Roll the new plant back instead so
  the confirm is all-or-nothing (a fresh plant has no lots/plantings, so
  the delete is safe; log-and-continue on cleanup failure).
- Dedup: packetLotRequest and seedLotCreateRequest shared every lot
  field. Extract a seedLotFields base both use. validCategory now reuses
  plantCategories. EffectiveConfig resolves agent+vision from one
  settings-row read instead of two.
- capabilities swallowed an EffectiveVision error silently; log it.
- vision test hand-copied Extract's body (drift risk). Split generate()
  out of Extract so the hermetic test drives the real request builder.

Tests: rollback-on-lot-failure (service), oversized->413 (api).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

internal/api/api.go

@@ -212,7 +212,12 @@ func (h *handlers) capabilities(c *gin.Context) {
 	// configured, resolvable vision model + a key. Read per-request so a settings
 	// change is reflected on the next poll, same as agent.
 	vision := false
-	if vis, err := h.svc.EffectiveVision(c.Request.Context()); err == nil {
+	if vis, err := h.svc.EffectiveVision(c.Request.Context()); err != nil {
+		// A read fault here means the DB is unhappy; report vision off (safe: the
+		// UI just hides a button) but don't do it silently — the same best-effort
+		// settings reads elsewhere log rather than swallow.
+		slog.Error("api: could not resolve vision settings for capabilities", "error", err)
+	} else {
 		vision = vis.Ready()
 	}
 	c.JSON(http.StatusOK, gin.H{"agent": h.agent.get() != nil, "vision": vision})