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/vision/vision.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 
 	"gitea.stevedudenhoeffer.com/steve/majordomo"
+	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
 
 	"gitea.stevedudenhoeffer.com/steve/pansy/internal/agentmodel"
 )
@@ -57,6 +58,14 @@ func Extract(ctx context.Context, apiKey, modelSpec string, jpeg []byte) (SeedPa
 	if err != nil {
 		return SeedPacket{}, err
 	}
+	return generate(ctx, model, jpeg)
+}
+
+// generate makes the actual one-shot call against an already-resolved model. It
+// is split from Extract on purpose: the hermetic test drives THIS function with a
+// fake model, so the prompt, the derived schema and the image part it builds are
+// the real ones the live path uses — not a hand-copied double that could drift.
+func generate(ctx context.Context, model llm.Model, jpeg []byte) (SeedPacket, error) {
 	return majordomo.Generate[SeedPacket](ctx, model, majordomo.Request{
 		Messages: []majordomo.Message{
 			majordomo.UserParts(