fix(security): remove hardcoded Flask secret key

Replace hardcoded Flask secret key with environment variable to
prevent session hijacking and CSRF attacks.

Changes:
- Load secret key from TISBACKUP_SECRET_KEY environment variable
- Fall back to cryptographically secure random key using secrets module
- Log warning when random key is used (sessions won't persist)
- Add environment variable example to README.md Docker Compose config
- Add setup instructions in Configuration section

Security improvements:
- Eliminates hardcoded secret in source code
- Uses secrets.token_hex(32) for cryptographically strong random generation
- Sessions remain secure even without env var (though won't persist)
- Prevents session hijacking and CSRF bypass attacks

Documentation:
- Update README.md with TISBACKUP_SECRET_KEY setup instructions
- Include command to generate secure random key
- Update SECURITY_IMPROVEMENTS.md with implementation details
- Mark hardcoded secret key issue as resolved

Setup:
```bash
# Generate secure key
python3 -c "import secrets; print(secrets.token_hex(32))"

# Set in environment
export TISBACKUP_SECRET_KEY=your-key-here
```

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

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

SECURITY_IMPROVEMENTS.md

@@ -104,26 +104,65 @@ import subprocess
 
 **Impact:** Cleaner namespace, easier to track dependencies
 
-## Remaining Security Issues (Critical - Not Fixed)
+### 7. Fixed Hardcoded Secret Key
+**Files Modified:** [tisbackup_gui.py:67-79](tisbackup_gui.py#L67), [README.md](README.md)
 
-### 1. **Hardcoded Secret Key** ([tisbackup_gui.py:64](tisbackup_gui.py#L64))
+**Before:**
 ```python
 app.secret_key = "fsiqefiuqsefARZ4Zfesfe34234dfzefzfe"
 ```
-**Recommendation:** Load from environment variable or secure config file
 
-### 2. **No Authentication on Flask Routes**
+**After:**
+```python
+SECRET_KEY = os.environ.get("TISBACKUP_SECRET_KEY")
+if not SECRET_KEY:
+    import secrets
+    SECRET_KEY = secrets.token_hex(32)
+    logging.warning(
+        "TISBACKUP_SECRET_KEY environment variable not set. "
+        "Using a randomly generated secret key. "
+        "Sessions will not persist across application restarts. "
+        "Set TISBACKUP_SECRET_KEY environment variable for production use."
+    )
+app.secret_key = SECRET_KEY
+```
+
+**Changes:**
+- Reads secret key from `TISBACKUP_SECRET_KEY` environment variable
+- Falls back to cryptographically secure random key if not set
+- Logs warning when using random key (sessions won't persist across restarts)
+- Uses Python's `secrets` module for cryptographically strong random generation
+- Updated README.md with setup instructions
+
+**Setup Instructions:**
+```bash
+# Generate a secure secret key
+python3 -c "import secrets; print(secrets.token_hex(32))"
+
+# Set in Docker Compose (compose.yml)
+environment:
+  - TISBACKUP_SECRET_KEY=your-generated-key-here
+
+# Or export in shell
+export TISBACKUP_SECRET_KEY=your-generated-key-here
+```
+
+**Security Impact:** Eliminates hardcoded secret in source code, prevents session hijacking and CSRF attacks
+
+## Remaining Security Issues (Critical - Not Fixed)
+
+### 1. **No Authentication on Flask Routes**
 All routes are publicly accessible without authentication.
 
 **Recommendation:** Implement Flask-Login or similar authentication
 
-### 3. **Insecure SSH Host Key Policy** ([libtisbackup/common.py:649](libtisbackup/common.py#L649))
+### 2. **Insecure SSH Host Key Policy** ([libtisbackup/common.py:649](libtisbackup/common.py#L649))
 ```python
 ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
 ```
 **Recommendation:** Use proper host key verification with known_hosts
 
-### 4. **Command Injection in Legacy Code**
+### 3. **Command Injection in Legacy Code**
 Multiple files still use `subprocess.call(shell_string, shell=True)` and `subprocess.Popen(..., shell=True)`:
 - [libtisbackup/common.py:128](libtisbackup/common.py#L128)
 - [libtisbackup/common.py:883](libtisbackup/common.py#L883)