docs(dev): test-running policy + media-fixture rules; ignore testdata

Integration tests require the containerized dev DB and bind-mounted
paths — a bare host `go test ./cmd/server/tests` fails on database
auth on every machine not running the compose stack (the old doc
prescribed exactly that flow). development.md now documents the
container runner and the iteration rule: run ONLY your own tests via
a filtered container invocation while working, and reserve full-suite
runs for pre-commit validation — several tests read live shared state
(system_settings, real library folders, the periodic backup scan's
websocket broadcasts) and flap on data or timing a change never
touched.

Binary media fixtures are never committed: internal/services/
testdata/ is gitignored by policy (the audio metadata tests read
real-format binaries that ffmpeg cannot synthesize — Audible freeform
atoms — so they live untracked on the machines that run the suite).
The new "Media fixtures" section carries the ffmpeg recipes for every
supported container, tag writing, embedded covers and ffmetadata
chapters, plus the manual end-to-end recipe for the live system
(one book per subfolder — a directory with ≥2 audio files ingests as
one folder book).

.gitignore

@@ -82,3 +82,7 @@ session-*.md
 
 # Local prod-export scratch: personal annotation/reading data, never commit
 /export/
+
+# Binary media fixtures: never commit (generate with ffmpeg at test time,
+# see docs/contributing/development.md → Media fixtures)
+internal/services/testdata/