Test the image proxy flow end to end (closes #80)

TestImageProxyFlow in internal/server starts the database, handlers and
middleware from the constructors pixad uses, in an fx app with a fresh
state directory, and replaces only the upstream origin with a local test
server, reached through the real fetcher by its new DialContext. For a
resize with a change to JPEG and for orig, the first request answers 200
with the right content type, decoded size and X-Pixa-Cache MISS; the
second answers HIT with the same image while the upstream has had one
request; the source and the converted image are then on disk with their
rows in SQLite. TODO.md records it and drops the item from Future Steps.

Model: opus-5-5

TODO.md

@@ -31,6 +31,21 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 an integration test of the image proxy flow (closes #80):
+  `TestImageProxyFlow` in `internal/server` starts the database, handlers and
+  middleware from the constructors `pixad` uses, with a fresh state directory,
+  and replaces only the upstream origin with a local test server. For a resize
+  with a change to JPEG and for `orig`, the first request goes through the
+  router, the real fetcher, libvips, the disk cache and SQLite and answers 200
+  with the right content type and size and `X-Pixa-Cache: MISS`; the second
+  answers `HIT` with the same image and the upstream has had one request; the
+  source and the converted image are then in `cache/sources` and
+  `cache/variants`, with their rows in `source_content`, `source_metadata` and
+  `variant_content`. Two optional fields make this possible, which `pixad` does
+  not set and the config file and environment cannot:
+  `httpfetcher.Config.DialContext` connects in place of the dialer that refuses
+  internal addresses, the URL and redirect checks still running, and
+  `handlers.Params.Fetcher` replaces the fetcher the handlers build.
 - 2026-10-04 fewer files in the repository root (closes #97):
   `config.example.yml` moved unchanged to `configs/config.example.yml`, and
   `README.md`, the comments in `internal/config/config.go` and the startup error
@@ -573,5 +588,4 @@ P2: security: referer blacklist
     - optional Sentry error reporting
     - comprehensive request logging
     - Prometheus performance metrics
-    - integration tests for the image proxy flow
     - load tests to verify the 1k to 5k req/s target