feat: Add BuildKit pip cache, unlock fastapi pin, mark Locking TODO complete

- Dockerfile: enable BuildKit syntax, use --mount=type=cache for pip to speed up rebuilds
- requirements.txt: relax fastapi==0.115.5 → fastapi>=0.115.5
- TODO: mark Locking task as complete

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 # Aether API - FastAPI + Gunicorn
 # Using tiangolo's optimized FastAPI image
 FROM tiangolo/uvicorn-gunicorn-fastapi:python3.11
@@ -16,7 +17,8 @@ RUN apt-get update; \
 # Install Python requirements
 # This file is now located in the project root
 COPY requirements.txt /tmp/requirements.txt
-RUN pip install --no-cache-dir -r /tmp/requirements.txt
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install -r /tmp/requirements.txt
 
 # Create a reference of actual installed versions
 RUN pip freeze >> /tmp/aether_fastapi_requirements_current.txt