feat(lookup): a cache TTL of 0 switches the cache off

Wanted for the case where the answers are moving: an operator correcting their
own QRZ record, or chasing a DXpedition whose page changes during the operation,
otherwise waits out thirty days before OpsLog will ask again. Clearing the cache
by hand works once; this is the setting for a whole session.

Nothing is read from it and nothing is written to it — rows stored while it is
off would only sit there going stale, waiting for the day it comes back on.
Switching off is NOT clearing: what it already holds stays, and the Clear cache
button remains the way to throw that away.

Two distinctions the code now has to keep, both load-bearing:

An EXPLICIT stored zero is off; an ABSENT key is the thirty-day default. Every
operator who has never opened this setting has nothing stored, and reading that
blank as a zero would silently switch the cache off for all of them.

The CONSTRUCTOR's zero is still the default, not off. At startup the settings
have not been read yet, and beginning with no cache would hammer the provider
for the first seconds of every launch. Only SetTTL, called once the operator's
settings are known, can switch it off.

A negative lifetime is meaningless and is ignored rather than rounded into
either meaning.

The input had to change too: it derived its value from the stored number on
every keystroke, so the box could not be emptied — and 0 was unreachable
outright, since parseInt('0') || 30 is 30.

internal/lookup/lookup.go

@@ -432,11 +432,21 @@ func fillFromDXCC(r *Result, dxcc DXCCResolver) bool {
 // ----- Cache -----
 
 // Cache is a SQLite-backed cache of lookup results with a TTL.
+//
+// A ttl of zero means NO CACHE: every lookup goes to the provider. That is a
+// real thing to want — an operator correcting their own QRZ record, or chasing
+// a DXpedition whose page changes during the operation, otherwise waits out the
+// cache before OpsLog will look again.
 type Cache struct {
 	db  *sql.DB
 	ttl time.Duration
 }
 
+// NewCache builds the cache. A ttl of zero here is the CONSTRUCTOR default
+// (thirty days), not "off": at startup the settings have not been read yet, and
+// starting with no cache would hammer the provider for the first seconds of
+// every launch. Switching it off is a decision the operator makes, through
+// SetTTL, once their settings are known.
 func NewCache(db *sql.DB, ttl time.Duration) *Cache {
 	if ttl <= 0 {
 		ttl = 30 * 24 * time.Hour
@@ -444,15 +454,25 @@ func NewCache(db *sql.DB, ttl time.Duration) *Cache {
 	return &Cache{db: db, ttl: ttl}
 }
 
-// SetTTL updates the cache TTL (e.g. when user changes settings).
+// SetTTL updates the cache lifetime.
+//
+// ZERO switches the cache OFF — nothing is read from it and nothing is written
+// to it. A NEGATIVE value is meaningless and is ignored, rather than being
+// rounded into one of the two meanings above.
 func (c *Cache) SetTTL(ttl time.Duration) {
-	if ttl > 0 {
+	if ttl >= 0 {
 		c.ttl = ttl
 	}
 }
 
+// Enabled reports whether anything is being cached at all.
+func (c *Cache) Enabled() bool { return c != nil && c.ttl > 0 }
+
 // Get returns the cached result if present and not expired.
 func (c *Cache) Get(ctx context.Context, callsign string) (Result, bool) {
+	if !c.Enabled() {
+		return Result{}, false
+	}
 	row := c.db.QueryRowContext(ctx, `
         SELECT callsign, name, qth, address, state, cnty, country, grid,
                lat, lon, dxcc, cqz, ituz, cont, email, qsl_via, image_url,
@@ -519,6 +539,11 @@ func (c *Cache) Get(ctx context.Context, callsign string) (Result, bool) {
 // Put upserts a lookup result. fetched_at is generated in Go (NowISO) so the
 // INSERT is backend-agnostic; the conflict tail is dialect-specific.
 func (c *Cache) Put(ctx context.Context, r Result) error {
+	if !c.Enabled() {
+		// Nothing reads it, so writing would only grow the table — and leave
+		// stale rows waiting for the day the cache is switched back on.
+		return nil
+	}
 	updateCols := []string{
 		"name", "qth", "address", "state", "cnty",
 		"country", "grid", "lat", "lon",