fix: backend Dockerfile missing COPY for canary/ package (a4eec13 breaks startup)

[vybe]

Summary

Commit a4eec13 (feat(#411): canary invariant harness Phase 1) added src/backend/canary/ but did not add a corresponding COPY line to docker/backend/Dockerfile. The backend container fails to start with:

File "/app/routers/canary.py", line 21, in <module>
    from canary import INVARIANTS
ModuleNotFoundError: No module named 'canary'

Root Cause

docker/backend/Dockerfile copies source directories explicitly. The new canary/ package was not added:

# Existing copies:
COPY ../../src/backend/routers /app/routers/
COPY ../../src/backend/services /app/services/
COPY ../../src/backend/db /app/db/
# Missing:
COPY ../../src/backend/canary /app/canary/   ← not present in a4eec13

Fix

Add to docker/backend/Dockerfile after the existing COPY block:

COPY ../../src/backend/canary /app/canary/

Impact

construct instance attempted to deploy a4eec13, backend failed to start, rolled back to 7b95c46. No data loss.

Reproduction

git checkout a4eec13
docker compose build --no-cache backend
docker compose up -d backend
docker logs trinity-backend   # ModuleNotFoundError: No module named 'canary'

[dolho]

Closing — already fixed by commit 62cc7495 ("fix(docker): add missing COPY for canary/ package in backend Dockerfile (#768)"). Same bug, different PR number. The current docker/backend/Dockerfile (line 57) has COPY ../../src/backend/canary /app/canary/ and the chmod block (lines 65–66) includes the package. Reopen if the breakage came back.

[dolho]

Correction on the earlier comment — premature close. Fix 62cc7495 is merged to dev only, not yet to main. Reopening + relabeling status-in-dev per the team rule that issues close only on main merge (i.e. at release cut).

[vybe]

Shipped in v0.6.0 (released via #998).