docs(03): create phase plan

.planning/phases/03-first-vertical-slice-genres-end-to-end/03-CONTEXT.md

@@ -23,7 +23,7 @@ Not in this phase: any other route (including `POST genres`), register/login/ref
 - **D-03:** Active-collection resolution lives in the `golem15.fonoteka` plugin as a small `ActiveCollection` service porting only the default path of `ActiveCollectionResolver.php` (stored active collection, else the user's own). Handlers call it the way PHP controllers call `context()`. It is not a framework middleware; the pipeline's org-context stage stays a generic slot. Phase 12 extends the service with switching and editor membership.
 - **D-04:** The handler is ported whole: `?non_empty` is validated (`nullable`, in `0|1`), bad values return PHP's 422 envelope (`{"error":"Validation failed","errors":{...}}`), and `non_empty=1` filters to genres with a positive count. No new fixtures are recorded for these; they are covered by Go tests with the expected bodies read from the PHP source.
 - **D-05:** The non-zero count path and access scoping are proven by a testcontainers integration test in `fonoteka.go` alongside the parity test: insert albums for alice in her active collection, albums in a collection she cannot access, assert per-genre counts, `non_empty=1` filtering and that foreign albums are not counted. The parity fixture stays exactly as recorded; the PHP recorder is not re-run in this phase.
-- **D-06:** The Postgres equivalent of `PolishOrder` is a research item with a user confirmation at plan review. The researcher reads `PolishOrder.php` and compares an ICU `pl` collation applied in the query, a column-level collation, and a database-level locale (including what testcontainers must match). Whatever is chosen lands as a reusable `lagoon` helper called at the same call site as PHP's `PolishOrder::apply`, because every later list endpoint needs it.
+- **D-06:** The Postgres equivalent of `PolishOrder` is a database-level Polish ICU locale, confirmed by the user at plan review on 2026-09-17 after comparing query-level, column-level, and database-level options. Create the application database with ICU locale `pl-PL` before migrations; configure testcontainers to create the same kind of database and fail startup if the connected database uses a different provider or locale. A reusable `lagoon` ordering helper is still called at the same handler call site as PHP's `PolishOrder::apply`, validating the allowed column and direction while relying on the database default. Every later list endpoint can use that helper.
 
 ### JWT guard
 - **D-07:** Verification is real; only issuing is throwaway. The guard validates HS256 with the algorithm pinned (no `alg` from the token header, `none` rejected), checks `exp` and `sub`, loads the user row by `sub` from the minimal `users` table, and places the user in the request context. A token for a missing user is rejected. Phase 7 replaces how tokens are minted and keeps this verifier.