E09: Product documentation backlog (#101)

delivery_backlog/stories/US09-03-architecture-overview.md

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+# US09-03 — Write the Architecture Overview
+
+Epic: [E09](../E09-documentation.md)
+
+As a developer or reviewer new to this repository, I want an architecture overview that
+explains what the pieces are and which rules they enforce, so that I can find the right
+module and not violate an invariant I never knew existed.
+
+## Acceptance criteria
+
+- A **context diagram**: the operator, the browser application, the API and worker, the
+  photo library, the Immich server, the vision provider, and the archive destination —
+  with the direction and nature of every interaction, including which ones leave the
+  machine.
+- A **runtime diagram**: the migrate/api/worker composition, the data volume, the library
+  bind mount, the SQLite database, and the process lock — showing what is shared and what
+  is exclusive.
+- A **module map**: every package under `photo_pipeline/` with its responsibility and the
+  boundary it must not cross (which modules may touch the filesystem, which may call an
+  external provider, which own schema).
+- **Key flows**, each as a diagram plus prose: the durable job lifecycle from enqueue to
+  recovery; the rename journal state machine including the rollback and manual-resolution
+  paths; the upload lifecycle through preflight, run, report ingestion, and verification.
+- **The invariants and where they are enforced**, each pointing at the module that owns
+  it: one writer at a time, the library process lock, the path policy and library roots,
+  `_IGNORE/` exclusion, verified EXIF before upload, safety decisions gating the vision
+  provider, and restart-safety of every mutation.
+- **The data model**: the tables, what identity means for an asset, and how a moved file
+  keeps its identity.
+- A short **history and donor** section: what was migrated out of the legacy CLIs, where
+  the archive and its ledger live, and why they are kept.
+- Diagrams are ```mermaid``` blocks so the source is diffable and Gitea renders them
+  natively.
+
+## Automated tests
+
+- Every package and top-level module under `photo_pipeline/` appears in the module map,
+  and every module named in the map exists — a new service cannot appear on the map's
+  blind side.
+- Every job state, journal state, and upload batch state named in the document exists in
+  the code, and every state the code defines is named in the document.
+- Every mermaid block parses (rendered in the browser test without an error node).
+- Every code path referenced by file name exists.
+
+## Dependencies
+
+- US09-01