Fail loudly on set-but-unparseable env config values (closes #80)

The config env helpers silently substituted the documented default
whenever a variable was set but could not be parsed, so a typo in an
operator-supplied value produced a running daemon with configuration
nobody asked for instead of a startup failure. `PORT=eighty` quietly
listened on 8080 and `DEBUG=ture` quietly disabled debug logging.

Defaults now apply only to variables that are unset or empty. Any
variable that is set but unparseable is a hard error that names the
key and the offending value and aborts startup through fx.

- add `envPositiveInt` with `ErrNonPositiveValue`, copied verbatim
  from the definition on the unmerged #87 so that rebasing after it
  lands is a delete-one-copy operation rather than a semantic merge
- remove `envInt` entirely; `PORT` is parsed by a new `envPort`,
  which adds the TCP upper bound (`ErrInvalidPort`, 1..65535)
- change `envBool` to return an error and parse with
  `strconv.ParseBool`, so `yes`, `on`, and typos are rejected rather
  than silently treated as false; callers are `DEBUG` and
  `MAINTENANCE_MODE`
- move env loading into `loadFromEnv`, with the environment check
  extracted to `resolveEnvironment` (also as #87 defines it), keeping
  `New` within the funlen budget

`envString` parses nothing and `envDuration` was already fail-loud,
so both are unchanged. A repo-wide audit of `os.Getenv`/`os.LookupEnv`
found no parse sites outside `internal/config`.

Tests cover each helper with a table (unset, valid, set-but-invalid)
plus `config.New`-level cases proving a bad `PORT`, `DEBUG`, or
`MAINTENANCE_MODE` aborts startup while unset variables still get
their defaults. README documents the fail-loud rule and the accepted
boolean spellings.

README.md

@@ -89,10 +89,27 @@ TTY detection, and security headers are always applied.
 | `PORT`                  | HTTP listen port                    | `8080`   |
 | `DATA_DIR`              | Directory for all SQLite databases  | `/var/lib/webhooker` |
 | `DEBUG`                 | Enable debug logging                | `false`  |
+| `MAINTENANCE_MODE`      | Serve the maintenance page          | `false`  |
 | `METRICS_USERNAME`      | Basic auth username for `/metrics`  | `""`     |
 | `METRICS_PASSWORD`      | Basic auth password for `/metrics`  | `""`     |
 | `SENTRY_DSN`            | Sentry error reporting DSN          | `""`     |
 
+#### Invalid values abort startup
+
+The defaults above apply **only** to variables that are unset (or set
+to an empty string). A variable that is set but cannot be parsed is a
+fatal configuration error: webhooker logs the offending variable and
+its value and refuses to start, rather than silently running with a
+substituted default. `PORT=eighty`, `DEBUG=ture`, and
+`RETENTION_SWEEP_INTERVAL=1 hour` all abort startup. `PORT` must
+additionally be a number in the range 1–65535.
+
+Boolean variables (`DEBUG`, `MAINTENANCE_MODE`) accept exactly the
+spellings Go's `strconv.ParseBool` accepts — `1`, `t`, `T`, `TRUE`,
+`true`, `True`, `0`, `f`, `F`, `FALSE`, `false`, `False` — and nothing
+else. `yes`, `on`, and `off` are rejected rather than quietly treated
+as false.
+
 On first startup, webhooker automatically generates a cryptographically
 secure session encryption key and stores it in the database. This key
 persists across restarts — no manual key management is needed.