add: DB-backed settings UI for live-editable tunables

Add a settings page (/settings) and API (/api/settings GET/PUT) to edit
encoding profiles (crf/preset per media type), lp, OMDb key, log retention,
and delete-originals from the browser, persisted to a settings table in
logs.db. config.yaml seeds the store on first run; afterwards the DB is the
source of truth (paths.* and http_addr stay config-only). Each job snapshots
the current settings, so changes apply to the next encode with no restart.

PUT validates ranges (crf 0-63, preset 0-13, lp >=0, retention >=1) before
persisting. No auth, by design for now (isolated network) — see #2.

Closes #1

internal/server/index.html

@@ -27,6 +27,8 @@
   .wordmark { font-family:var(--mono); font-weight:600; font-size:1.4rem; letter-spacing:-.02em; }
   .wordmark .prompt { color:var(--dim); }
   .wordmark .vibe { color:var(--amber); }
+  .navlink { font-family:var(--mono); font-size:.78rem; color:var(--muted); text-decoration:none; }
+  .navlink:hover { color:var(--amber); }
   .live { margin-left:auto; display:flex; align-items:center; gap:.5rem; font-family:var(--mono); font-size:.74rem; color:var(--muted); text-transform:uppercase; letter-spacing:.1em; }
   .dot { width:9px; height:9px; border-radius:50%; background:var(--dim); }
   .dot.ok { background:var(--green); box-shadow:0 0 0 0 rgba(92,201,139,.6); animation:pulse 2s infinite; }
@@ -118,6 +120,7 @@
 
 <header>
   <span class="wordmark"><span class="prompt">$ </span>av<span class="vibe">1</span>dae</span>
+  <a href="/settings" class="navlink">settings</a>
   <span class="live"><span class="dot" id="dot"></span><span id="livetext">connecting</span></span>
 </header>