Seed-packet capture: vision model, extraction, catalog match, create (backend) (#81)

Photograph a seed packet → it fills in the plant and the purchase. This is the
backend; the scan UI is a follow-up PR.

Vision model config (mirrors the agent model from #79):
- Migration 0011 adds instance_settings.vision_model; PANSY_VISION_MODEL is the
  env default. Precedence Settings → env → empty; the KEY stays in the env.
- EffectiveVision resolves it; /capabilities advertises "vision" only when a
  model + key are configured, so the UI offers the scan button only when it works.

Extraction is one-shot, NOT an agent loop (internal/vision):
- majordomo.Generate[SeedPacket] derives a JSON schema from the struct tags and
  hands the image to the vision model; it can't call a tool, so it can't touch
  the garden — it only reads a picture and returns data. Numeric fields are
  pointers, so a field the packet doesn't print comes back nil, not a made-up 0.
- Hermetic test: majordomo's fake provider returns canned packet JSON and
  Generate unmarshals it, image + derived schema included. No live model.

The image is normalized to JPEG at the upload boundary (imagenorm from #80),
which is where an iPhone HEIC becomes readable — majordomo's media path can't
decode HEIC. imagenorm now links into the binary (~7 MB, the cost #80 deferred).

The hard part is catalog matching, not OCR (internal/service/seed_packet.go):
- A wrong auto-match splits a variety's seed-lot history across duplicate rows,
  so the service NEVER auto-creates. matchPlants surfaces RANKED candidates
  (exact name → variety-in-name → same species, conservative and name-based),
  the user confirms, and CreateFromPacket makes the plant (new or existing) + the
  lot. Exactly one of plantId/newPlant, refused otherwise.
- Plants/lots aren't in the undo history (catalog/inventory), so no change set.
- The extractor is injectable (service.WithPacketExtractor) so ExtractSeedPacket
  and the /scan endpoint test end to end against a fake, no live model.

Endpoints: POST /seed-lots/scan (multipart image → proposal, reads only; extends
the read deadline for a slow phone upload, caps the body, maps too-large/unreadable
to clear statuses) and POST /seed-lots/from-packet (confirmed proposal → 201).

Docs: README (PANSY_VISION_MODEL), DESIGN (routes + the decision and why the
model can't touch the garden).

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

README.md

@@ -70,8 +70,9 @@ The garden assistant reads three more. Setting none of them leaves the assistant
 | `OLLAMA_CLOUD_API_KEY`  | *(empty)*                      | Ollama Cloud API key. Without it the assistant is off, not broken. This is the one agent value that stays in the environment — it is **never** stored in the database or editable in Settings. |
 | `PANSY_AGENT_MODEL`     | `ollama-cloud/glm-5.2:cloud`   | Default model spec, passed verbatim to `majordomo.Parse` — a comma-separated list is a failover chain, e.g. `ollama-cloud/glm-5.2:cloud,ollama-cloud/kimi-k2.6:cloud`. An admin can override this per-instance in **Settings** without a redeploy; a blank Settings value inherits this. |
 | `PANSY_AGENT_ENABLED`   | on when a key is present       | Default on/off for the assistant. Also overridable in Settings (which can inherit this default). |
+| `PANSY_VISION_MODEL`    | *(empty)*                      | Default model for **seed-packet capture** (photograph a packet → it fills in the plant + purchase). A *vision-capable* model (the chat model may not be). Empty = the feature isn't offered. Runs against the same `OLLAMA_CLOUD_API_KEY`, and is overridable in Settings. |
 
-The model and enabled flag can be changed at runtime by an admin under **Settings** (the gear appears in the nav for admins) — the change swaps the live assistant with no restart. The env vars above are the defaults an untouched instance uses, and the API key is intentionally not among the runtime-editable settings: a secret in the database would land in every backup. Precedence for the model and enabled flag is **Settings value, if set → env var → built-in default**.
+The agent model + enabled flag, and the vision model, can be changed at runtime by an admin under **Settings** (the gear appears in the nav for admins) — an agent change swaps the live assistant with no restart. The env vars above are the defaults an untouched instance uses, and the API key is intentionally not among the runtime-editable settings: a secret in the database would land in every backup. Precedence is **Settings value, if set → env var → built-in default**.
 
 The assistant acts without asking first, which is only reasonable because every turn is one undoable change set — see the History panel in the editor.