Dockerfile: copy the shared backend crate into the image build

Dockerfile

@@ -54,12 +54,18 @@ COPY apps/tracker/backend/Cargo.toml apps/tracker/backend/Cargo.toml
 COPY apps/tracker/integration/Cargo.toml apps/tracker/integration/Cargo.toml
 COPY apps/party/backend/Cargo.toml apps/party/backend/Cargo.toml
 COPY services/transcoder/Cargo.toml services/transcoder/Cargo.toml
+# EVERY workspace member's manifest has to be here, not just the ones being built:
+# cargo parses the whole workspace before it builds any of it, and a path
+# dependency it cannot read is a hard error ("failed to load manifest for
+# workspace member"). scene-backend is both — a member and the crate all three
+# backends depend on.
+COPY crates/scene-backend/Cargo.toml crates/scene-backend/Cargo.toml
 # Stub sources so cargo can parse + warm the dep cache for every shipped crate.
 # integration is test-only (never built here) but its manifest must parse → stub lib.
 # tracker-backend declares extra bins (tracker-migrate, tracker-enrich — offline
 # tools); their src files must exist for cargo to resolve the manifest even though
 # only the server bin ships.
-RUN mkdir -p apps/tracker/backend/src/bin apps/tracker/integration/src apps/party/backend/src services/transcoder/src \
+RUN mkdir -p apps/tracker/backend/src/bin apps/tracker/integration/src apps/party/backend/src services/transcoder/src crates/scene-backend/src \
     && printf 'fn main() {}\n' > apps/tracker/backend/src/main.rs \
     && : > apps/tracker/backend/src/lib.rs \
     && printf 'fn main() {}\n' > apps/tracker/backend/src/bin/migrate.rs \
@@ -68,9 +74,20 @@ RUN mkdir -p apps/tracker/backend/src/bin apps/tracker/integration/src apps/part
     && printf 'fn main() {}\n' > apps/party/backend/src/main.rs \
     && : > apps/party/backend/src/lib.rs \
     && printf 'fn main() {}\n' > services/transcoder/src/main.rs \
+    && : > crates/scene-backend/src/lib.rs \
     && xx-cargo build --release -p tracker-backend -p party-backend -p scene-transcoder
 
-FROM workspace-deps AS tracker-backend-build
+# The shared crate's REAL source, in one stage the three backend stages extend.
+# It has to land before any of them build: with only the stub lib in place they
+# would compile against an empty scene-backend and fail on every item they use.
+# Here rather than repeated per backend so a change to it rebuilds all three from
+# the same layer.
+FROM workspace-deps AS shared-build
+ARG TARGETPLATFORM
+COPY crates/scene-backend/src ./crates/scene-backend/src
+RUN touch crates/scene-backend/src/lib.rs
+
+FROM shared-build AS tracker-backend-build
 ARG TARGETPLATFORM
 COPY apps/tracker/backend/src ./apps/tracker/backend/src
 # `touch` so cargo notices the stub→real source swap (shared target dir → only
@@ -80,14 +97,14 @@ RUN touch apps/tracker/backend/src/main.rs apps/tracker/backend/src/lib.rs \
     && xx-cargo build --release -p tracker-backend --bin tracker-backend \
     && cp target/*/release/tracker-backend /tracker-backend
 
-FROM workspace-deps AS party-backend-build
+FROM shared-build AS party-backend-build
 ARG TARGETPLATFORM
 COPY apps/party/backend/src ./apps/party/backend/src
 RUN touch apps/party/backend/src/main.rs apps/party/backend/src/lib.rs \
     && xx-cargo build --release -p party-backend \
     && cp target/*/release/party-backend /party-backend
 
-FROM workspace-deps AS transcoder-build
+FROM shared-build AS transcoder-build
 ARG TARGETPLATFORM
 COPY services/transcoder/src ./services/transcoder/src
 RUN touch services/transcoder/src/main.rs \