Build the image pipeline in pixad's fx graph (closes #313)

internal/app now provides the cache, the fetcher, the processor, the
image service, the session manager and the encrypted URL generator, and
handlers.New receives them built. database.New opens the database, so
these parts can read it while fx builds them; the cache's stop hook
still runs before the database's.

Each setting's default is declared in internal/config, the default
cache_max_bytes included, which internal/app works out before it builds
the cache. The cache's MaxBytes is the one value for the disk cache
limit, and 0 turns the disk cache off. The fetcher and the processor
take explicit values. The server tests build the pipeline themselves,
as they cannot import internal/app.

Model: opus-5-5

TODO.md

@@ -30,6 +30,30 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-26 pixad's fx graph builds the image pipeline (closes #313).
+  `database.New` now opens the database, so the parts below can read it while
+  they are built; its stop hook still runs after the cache's. The cache's
+  `MaxBytes` is the one value for the disk cache limit, and 0 turns the disk
+  cache off. For an omitted `cache_max_bytes`, `internal/app` works out
+  `config.DefaultCacheMaxBytes` before it builds the cache. The server tests
+  build the pipeline with `newImagePipeline`, as they cannot import
+  `internal/app`.
+    - Built by the providers in `internal/app/providers.go`, and given to
+      `handlers.New`:
+        - the cache
+        - the fetcher
+        - the processor
+        - the image service
+        - the session manager
+        - the encrypted URL generator
+    - Removed:
+        - `initImageService` and `handlers.Params.Fetcher`
+        - `CacheConfig.UseDefaultMaxBytes`, `CacheConfig.DisableDiskCache` and
+          `Cache.disabled`
+        - `httpfetcher.DefaultConfig()` and the fetcher's copies of setting
+          defaults
+        - `imageprocessor.DefaultMaxInputBytes`, and the processor's defaults
+          for a zero `MaxInputBytes` or `MaxConcurrentProcessing`
 - 2026-10-26 the code nothing in production called is gone (closes #312). The
   handlers tests sign their image URLs with `generateSignedURL`, a test helper
   that writes them as the removed generator did. Removed: