feat: add config module with environment variable handling

Implements Step 1.1 config.py with Pydantic Settings:
- PORT (default 3366), DB_PATH, BASE_PATH
- AUTH_HEADER_NAME, TRUSTED_PROXY_IPS (comma-separated)
- CSRF_SECRET (required), CSRF_COOKIE_NAME
- SEED_ON_START, LOG_LEVEL (validated), METRICS_ENABLED

Includes 28 TDD tests covering defaults, overrides, and validation.
Also fixes pytest pythonpath configuration.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

PLAN.md

@@ -7,12 +7,12 @@ Check off items as completed. Each phase builds on the previous.
 ## Phase 1: Foundation
 
 ### Step 1.1: Project Structure & Configuration
-- [ ] Create Python package structure (`src/animaltrack/`)
-- [ ] Create `pyproject.toml` with dependencies
-- [ ] Create `flake.nix` with dev environment
-- [ ] Create `.envrc`, `.gitignore`
-- [ ] Implement `config.py` with all env vars from spec §18
-- [ ] Write tests for config loading and validation
+- [x] Create Python package structure (`src/animaltrack/`)
+- [x] Create `pyproject.toml` with dependencies
+- [x] Create `flake.nix` with dev environment
+- [x] Create `.envrc`, `.gitignore`
+- [x] Implement `config.py` with all env vars from spec §18
+- [x] Write tests for config loading and validation
 - [ ] **Commit checkpoint**
 
 ### Step 1.2: Database Connection & Pragmas