mAi: #3 - Replicate adapter, mai.imagen_usage cost-tracking, usage CLI

Implements the Replicate API backend (FLUX schnell / FLUX dev) per ImaGen
issue #3:

- internal/backend/replicate.go — Backend adapter. Supports model
  refs as "owner/name" (uses /v1/models/{owner}/{name}/predictions) and
  "owner/name:hash" (uses /v1/predictions with explicit version). Polls
  /v1/predictions/{id} every 500ms with model-aware timeout (60s schnell,
  120s dev). Resilience: 401 names api_token_env, 429 with exp backoff
  up to 3 retries (honours Retry-After), 5xx retries once, image
  download retries once on transient failure.
- internal/backend/replicate_pricing.go — hardcoded per-image USD rates
  for known FLUX models, snapshotted from replicate.com/pricing with a
  refresh TODO.
- internal/backend/replicate_test.go — mocked-HTTP unit tests covering
  happy path (model + version-pinned), 401, 429 retry policy, failed
  prediction, poll timeout, image-download retry, ctx cancel, BackendOpts
  passthrough, default_steps, aspect-ratio reduction, sha256 prompt hash.
- internal/usage/usage.go — Supabase REST sink + read-side query for
  mai.imagen_usage. Adapter writes are best-effort: failures warn but
  the image still lands.
- cmd/imagen/usage.go — `imagen usage [--since DATE] [--raw]` reads
  the table and prints a tab-aligned grouped or raw table with totals.
- cmd/imagen/backends.go — instances of type=replicate now report
  "ok" or "not configured (set REPLICATE_API_TOKEN)" depending on env.
- internal/config/config.go — sample adds flux-schnell-replicate +
  flux-dev-replicate; default_backend stays flux-schnell-local.
- Supabase migration mai.imagen_usage (id, created_at, backend, model,
  seed, prompt_hash, latency_ms, cost_usd_estimate, caller) + indexes
  on (created_at DESC) and (caller). The raw prompt is never stored.

Caller identity resolves from MAI_FROM_ID, then the tmux pane's
@mai-name option, mirroring the maimcp identity logic. Prompt hash is
sha256 of the user-facing prompt; raw prompt never reaches the table.

docs/usage.md

@@ -91,3 +91,26 @@ API-backed adapters read tokens from env vars referenced by the config
 export REPLICATE_API_TOKEN=...
 imagen generate "a cat" --backend flux-dev-replicate
 ```
+
+## Cost-tracking (Replicate)
+
+Successful generations through the Replicate adapter write one row to
+`mai.imagen_usage` on Supabase: backend, model, latency, per-image cost
+estimate, prompt sha256 hash (never the prompt itself), and the caller
+identity (resolved from `MAI_FROM_ID` or the tmux pane's `@mai-name`).
+
+The writer is best-effort. If `SUPABASE_URL` / `SUPABASE_SERVICE_KEY` are
+unset, or the database write fails, the image still lands and the CLI
+prints a warning to stderr.
+
+Inspect spend:
+
+```sh
+imagen usage                       # all rows, grouped by week + backend + model + caller
+imagen usage --since 2026-05-01    # only rows on/after a UTC date
+imagen usage --since 2026-05-01 --raw
+```
+
+Per-model rates live in `internal/backend/replicate_pricing.go` — they
+are snapshotted from <https://replicate.com/pricing> and refreshed on a
+quarterly cadence.