Add --version / version with baked-in git commit

ldflags inject version, commit, and build date; a version subcommand
also reports go version and platform. Ends the which-binary-is-this
guessing after deploys.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Makefile

@@ -1,7 +1,15 @@
 BINARY := rtsp-streamer
 PKG := ./cmd/rtsp-streamer
 BINDIR := bin
-LDFLAGS := -s -w
+
+# Build metadata baked into the binary (see `rtsp-streamer version`).
+VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
+COMMIT  ?= $(shell git rev-parse --short HEAD 2>/dev/null || echo none)
+DATE    ?= $(shell date -u +%Y-%m-%dT%H:%M:%SZ)
+LDFLAGS := -s -w \
+	-X main.version=$(VERSION) \
+	-X main.commit=$(COMMIT) \
+	-X main.date=$(DATE)
 
 # Install location. Override with `make install PREFIX=/opt`.
 PREFIX ?= /usr/local