refactor: reorganize CI/CD pipelines into separate workflow files

BREAKING CHANGE: Monolithic ci.yml split into focused pipeline files

## Pipeline Reorganization

Split single CI/CD pipeline into 7 specialized workflows:

1. **lint.yml** - Code quality checks (Black, Ruff, MyPy)
2. **test.yml** - Test suite with coverage reporting
3. **security.yml** - Security scanning (Bandit)
4. **build.yml** - Docker image builds and registry push
5. **deploy-staging.yml** - Staging environment deployment
6. **deploy-production.yml** - Production deployment (tags only)
7. **docs-generation.yml** - Scheduled documentation generation

## Benefits

- **Modularity**: Each pipeline has single responsibility
- **Performance**: Workflows run independently, faster feedback
- **Clarity**: Easier to understand and maintain
- **Flexibility**: Trigger pipelines independently
- **Debugging**: Isolated failures easier to diagnose

## Dockerfile Improvements

- Fix FROM AS casing (was 'as', now 'AS') in all Dockerfiles
- Resolves Docker build warnings
- Improves consistency across build files

## Documentation

- Added .gitea/workflows/README.md with:
  - Workflow descriptions and triggers
  - Dependency diagram
  - Environment variables reference
  - Troubleshooting guide
  - Best practices

## Migration Notes

- Old monolithic pipeline backed up as ci.yml.old
- All triggers preserved from original pipeline
- No changes to build or deploy logic
- Same environment variables and secrets required

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

deploy/docker/Dockerfile.api

@@ -1,5 +1,5 @@
 # Dockerfile for FastAPI API Service
-FROM python:3.12-slim as builder
+FROM python:3.12-slim AS builder
 
 WORKDIR /build
 

deploy/docker/Dockerfile.chat

@@ -1,5 +1,5 @@
 # Dockerfile for Chat Service
-FROM python:3.12-slim as builder
+FROM python:3.12-slim AS builder
 
 WORKDIR /build
 

deploy/docker/Dockerfile.frontend

@@ -1,6 +1,6 @@
 # Dockerfile for React Frontend
 # Build stage
-FROM node:20-alpine as builder
+FROM node:20-alpine AS builder
 
 WORKDIR /build
 

deploy/docker/Dockerfile.worker

@@ -1,5 +1,5 @@
 # Dockerfile for Celery Worker Service
-FROM python:3.12-slim as builder
+FROM python:3.12-slim AS builder
 
 WORKDIR /build