E09: Product documentation backlog (#101)

delivery_backlog/stories/US09-02-installation-manual.md

@@ -0,0 +1,46 @@
+# US09-02 — Write the Installation and Operations Manual
+
+Epic: [E09](../E09-documentation.md)
+
+As somebody installing this application for the first time, I want one manual that takes
+me from nothing to a running instance pointed at my photo library, so that I do not have
+to reconstruct the procedure from the README, the compose file, and the test suite.
+
+## Acceptance criteria
+
+- A **host installation** path: prerequisites and their versions (Python, exiftool,
+  immich-go, Playwright's browser for the test suite), virtual environment, dependency
+  install, `.env`, database migration, starting `serve` and `worker`, and how to verify
+  the install succeeded.
+- A **container installation** path: the published image, the compose file, the data
+  volume, the library bind mount and why it is mounted the way it is, published ports,
+  running behind a reverse proxy with `PHOTO_PIPELINE_ALLOWED_HOSTS` and
+  `PHOTO_PIPELINE_TRUSTED_PROXIES`, and the Portainer/webhook deployment already in use.
+- A **configuration reference**: every `PHOTO_PIPELINE_*` setting with its meaning,
+  default, accepted values, and whether it is a secret. Secrets are described, never
+  exemplified with a real-looking value.
+- A **first run checklist** that ends in a verified state: library discovered, worker
+  claiming jobs, readiness endpoint green, diagnostics clean.
+- **Operations**: upgrading (including what the migration backup does), backup, verify,
+  restore, pruning, diagnostics, and the log locations for each role.
+- **Troubleshooting**: each refusal an operator can hit at startup — every non-zero exit
+  code of the CLI, the trust-boundary refusal, the unmounted library root refusal, the
+  library lock being held, and a legacy CLI still running — with the cause and the fix.
+- The manual states the safety invariants an installer must not work around: one worker
+  writes, `_IGNORE/` is never read, EXIF is verified before upload, and the library lock
+  is authoritative.
+
+## Automated tests
+
+- Every `PHOTO_PIPELINE_*` setting documented exists as a field on `Config`, and every
+  field on `Config` is documented — both directions, so a new setting cannot ship
+  undocumented.
+- Every CLI command and flag named in the manual exists in the argument parser, and every
+  subcommand the parser accepts appears in the manual.
+- Every exit code documented is one the CLI can actually return.
+- No documented example value collides with a real secret pattern (the repository's own
+  credential scanner runs over `docs/`).
+
+## Dependencies
+
+- US09-01