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

Config tests, and the first-boot debug log test that builds a Config,
start from an empty environment: config.ClearEnvForTest unsets every
variable the process has and restores each when the test ends, so
nothing exported in the developer's shell changes a result.
TestEnvPositiveInt and TestEnvPort share one table runner, and
TestEnvPort checks that the bad value appears in each error.

Every out-of-range PORT now wraps ErrInvalidPort: zero, negatives,
above 65535, and numbers too large or too small for an int.

The README configuration table and the Settings page say that a
RETENTION_SWEEP_INTERVAL that does not parse, or is zero or negative,
fails 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) |
@@ -2924,7 +2924,8 @@ 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