docs: add ADR-023 and glossary entries for OCR metrics

ADR-023 captures why prometheus-fastapi-instrumentator was chosen,
the build_metrics(registry) factory pattern, and the test rebinding
seam. The glossary gains four ops-aligned terms — illegible word,
models-ready gauge, recognition vs segmentation accuracy — so the
metrics documentation in OBSERVABILITY.md has a vocabulary to lean on.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/GLOSSARY.md

@@ -80,6 +80,14 @@ _See also [DocumentStatus lifecycle](#documentstatus-lifecycle)._
 
 **Sütterlin** — A specific standardized style of Kurrent taught in German schools from 1915 to 1941.
 
+**Illegible word** — a word whose recognition confidence falls below the configured threshold; replaced with the literal token `[unleserlich]` in the rendered block text and counted in the `ocr_illegible_words_total` Prometheus counter.
+
+**Models-ready gauge** — the `ocr_models_ready` Prometheus gauge, flipped from `0` to `1` once the FastAPI lifespan startup has finished loading the Kraken model and the spell-checker. Used both for the `/health` endpoint and as the supervised signal for the `ocr_models_ready < 1 for 2m` alert.
+
+**Recognition model accuracy** — the accuracy reported by `ketos train` for the recognition (text-line) model, exposed as `ocr_model_accuracy{kind="recognition"}`. Sourced from `_parse_best_checkpoint` on the highest-scoring checkpoint after training.
+
+**Segmentation model accuracy** — the accuracy reported by `ketos segtrain` for the baseline layout analysis (`blla`) model, exposed as `ocr_model_accuracy{kind="segmentation"}`. Distinct from recognition accuracy because the two models are trained and improved independently.
+
 ---
 
 ## Other Domain Terms

docs/adr/023-prometheus-instrumentator-and-metrics-registry-injection.md

@@ -0,0 +1,94 @@
+# ADR-023: Prometheus Instrumentator and Metrics Registry Injection
+
+## Status
+
+Accepted
+
+## Context
+
+Until issue #652 the OCR service exposed no `/metrics` endpoint. The
+observability stack already scrapes the Spring Boot backend's actuator
+endpoint, but it had nothing to scrape on the Python side. Without HTTP-
+and domain-level metrics from `ocr-service` we cannot answer questions
+like "what is the share of words rendered as `[unleserlich]`" or
+"is the training error rate above its budget" from Grafana.
+
+Two implementation requirements influenced the design:
+
+1. **Counter / gauge isolation in tests.** `prometheus_client` collectors
+   are module-level singletons keyed by name on the global `REGISTRY`.
+   Re-importing or naively re-instantiating them raises a duplicated-
+   collector error and cross-test state leaks (a `.inc()` in test A is
+   still readable by test B). A test harness needs a way to swap the
+   active container for a fresh per-test instance.
+
+2. **Minimal blast radius on the request path.** We did not want to
+   hand-instrument every endpoint with FastAPI middleware. The
+   `prometheus-fastapi-instrumentator` library already provides
+   `http_requests_total`, `http_request_duration_seconds`, and the
+   `/metrics` exposition route, all idiomatic Prometheus names.
+
+## Decision
+
+- Add `prometheus-fastapi-instrumentator==7.0.0` and pin its transitive
+  dependency `prometheus-client==0.25.0` explicitly in
+  `ocr-service/requirements.txt`.
+- Mount the instrumentator once at module load:
+  `Instrumentator(excluded_handlers=["/health", "/metrics"]).instrument(app).expose(app)`.
+  This adds `/metrics` and an HTTP-level dashboard surface without
+  changing any endpoint code.
+- Define every domain metric (`ocr_jobs_total`, `ocr_pages_total`,
+  `ocr_processing_seconds`, …) inside a `build_metrics(registry)`
+  factory in `ocr-service/metrics.py` that returns a frozen `OcrMetrics`
+  dataclass. Production code binds the container to the default
+  `REGISTRY` once: `metrics: OcrMetrics = build_metrics(REGISTRY)`.
+- Tests use a `fresh_metrics` fixture that builds a new
+  `CollectorRegistry()` per test and monkeypatches `main.metrics` with
+  a container bound to it. The endpoint code keeps reading
+  `metrics.<name>` without knowing whether it is talking to the global
+  registry or a per-test one.
+
+## Consequences
+
+**Positive**
+
+- One reusable factory captures the metric definitions; future metrics
+  go in one place.
+- Tests run with full counter isolation. Cross-test state leakage is
+  impossible because each test sees its own dataclass instance.
+- The instrumentator gives us `http_*` metrics for free, including a
+  Grafana-ready histogram that pairs with the Spring Boot one.
+
+**Negative**
+
+- One extra level of indirection: any test that asserts on metric
+  values must remember to monkeypatch `main.metrics`, not the registry
+  directly. Rebinding through the registry is harmless but useless —
+  the dataclass holds references to the original collectors.
+- `prometheus-client` is now pinned. Upgrading it requires an explicit
+  bump and re-checking the instrumentator's compatibility range.
+- `/metrics` is exposed unauthenticated and relies on the Docker
+  internal network for confidentiality. See
+  [docs/OBSERVABILITY.md §Internal-only endpoints](../OBSERVABILITY.md)
+  for the Caddy snippet that must be added if the service ever gets a
+  host-side port mapping.
+
+## Alternatives considered
+
+- **Hand-roll the `/metrics` endpoint.** Rejected: would have meant
+  duplicating what `prometheus-fastapi-instrumentator` ships, plus
+  middleware for the HTTP histograms.
+- **Skip the factory; pass `registry` as a function argument
+  everywhere.** Rejected: clutters every endpoint signature and breaks
+  the symmetry with the Spring Boot side, which also relies on a
+  process-global Micrometer registry.
+- **Use a `pytest` autouse fixture that resets `REGISTRY` between
+  tests.** Rejected: `prometheus_client` does not expose a clean
+  "unregister all" hook, and we would be relying on private APIs.
+
+## References
+
+- Issue: [#652](https://git.raddatz.cloud/marcel/familienarchiv/issues/652)
+- Library: <https://github.com/trallnag/prometheus-fastapi-instrumentator>
+- Code: `ocr-service/metrics.py`, `ocr-service/main.py`,
+  `ocr-service/test_metrics.py`