Isolate config tests from the shell; any out-of-range PORT is ErrInvalidPort (closes #94)

The config tests unset variables without restoring them and read whatever the developer's shell exported, so results could differ from one machine to the next. config.ClearEnvForTest, in internal/config/testing.go, unsets every variable in the process environment and, when the test ends, leaves it exactly as it found it; every config test and the first-boot test call it before setting their own. TestEnvPositiveInt and TestEnvPort share one table runner, and the port errors name the bad value. A PORT of zero, below zero, or too large to parse now matches ErrInvalidPort, as one above 65535 already did. The README and the Settings page say an unparseable or non-positive RETENTION_SWEEP_INTERVAL stops startup.

Model: opus-5-5

README.md

@@ -139,7 +139,7 @@ TTY detection, and security headers are always applied.
 | `METRICS_USERNAME`      | Basic auth username for `/metrics`. Must be set together with `METRICS_PASSWORD`; one without the other fails startup | `""`     |
 | `METRICS_PASSWORD`      | Basic auth password for `/metrics`. Must be set together with `METRICS_USERNAME`; one without the other fails startup | `""`     |
 | `SENTRY_DSN`            | Sentry error reporting DSN. Unset leaves error reporting off; a value the Sentry SDK cannot parse fails startup rather than serving with reporting silently off | `""`     |
-| `RETENTION_SWEEP_INTERVAL` | How often the retention reaper and archive sweeper run (Go duration, must be positive) | `1h` |
+| `RETENTION_SWEEP_INTERVAL` | How often the retention reaper and archive sweeper run (Go duration, must be positive). A value that does not parse, or is zero or negative, fails startup | `1h` |
 | `SESSION_IDLE_TIMEOUT`  | Idle session timeout (Go duration)  | `24h`    |
 | `RECEIVER_RATE_LIMIT`   | Receiver requests/minute per IP per entrypoint (10x that per IP across the route) | `120` |
 | `TRUSTED_PROXIES`       | CIDRs whose forwarded headers are trusted. A set value replaces the default. If any client can reach webhooker, or the proxy in front of it, from an RFC 1918 source address, set it to the proxy's address alone. See [Trusted proxies](#trusted-proxies) | `10.0.0.0/8,172.16.0.0/12,192.168.0.0/16` (RFC 1918) |
@@ -2942,7 +2942,8 @@ webhooker/
 │   ├── resetpw/
 │   │   └── resetpw.go         # `webhooker resetpw`: set an account's password, stopped deployments only
 │   ├── config/
-│   │   └── config.go          # Configuration loading from environment variables
+│   │   ├── config.go          # Configuration loading from environment variables
+│   │   └── testing.go         # ClearEnvForTest: an empty environment for one test
 │   ├── database/
 │   │   ├── base_model.go      # BaseModel with UUID primary keys
 │   │   ├── database.go        # GORM connection, migrations, admin seed