feat: move to python 3.14 baseline, patch remaining dependabot alerts

- requires-python >=3.14, pyupgrade --py314-plus, Dockerfile python:3.14-slim
- test matrix 3.14 only
- fastapi ^0.141.1 + starlette 1.6.0 (unblocks 5 starlette alerts)
- python-multipart ^0.0.31 (4 alerts)
- fix TemplateResponse for starlette 1.x request-first signature
- drop teyit hook: abandoned, incompatible with python 3.14

Dockerfile

@@ -1,5 +1,5 @@
 # Build stage
-FROM python:3.12-slim AS builder
+FROM python:3.14-slim AS builder
 
 WORKDIR /app
 
@@ -26,7 +26,7 @@ RUN pip install --upgrade pip setuptools wheel
 RUN pip install --no-cache-dir -r requirements.txt
 
 # Runtime stage
-FROM python:3.12-slim
+FROM python:3.14-slim
 
 # Set environment variables
 ENV PYTHONDONTWRITEBYTECODE=1
@@ -35,7 +35,7 @@ ENV PYTHONUNBUFFERED=1
 WORKDIR /app
 
 # Copy only the necessary files from the builder stage
-COPY --from=builder /usr/local/lib/python3.12/site-packages /usr/local/lib/python3.12/site-packages
+COPY --from=builder /usr/local/lib/python3.14/site-packages /usr/local/lib/python3.14/site-packages
 COPY --from=builder /usr/local/bin /usr/local/bin
 
 # Copy application code