feat(signup+admin): aliniere formular signup la landing + plan cerut, GDPR, control tier/trial in panou

Signup:
- /signup aliniat ca format la formularul din landing (campuri, etichete,
  placeholder-uri, select plan, checkbox GDPR, buton). Eticheta `name` = "Companie"
  (corecta: backendul salveaza nume de firma), uniform si in landing.
- Consimtamant GDPR validat server-side (functional, nu doar client-side) + salvat
  cu marca temporala (accounts.consent_at).
- Plan ales la signup salvat in accounts.requested_plan (intentie, NU drept): tier
  ramane sursa de adevar pentru gate-ul API; coloana pregateste integrarea platilor.
- landing: valorile `plan` = coduri tier (free/standard/pro/premium), data-plan
  sincronizat pe butoanele de pret; checkbox consimtamant primeste name.

Schema/DB:
- accounts: coloane noi requested_plan + consent_at (cu migrare aditiva in db.py).

Panou admin:
- Coloane noi: Plan curent (plan EFECTIV acum + zile trial ramase) si Plan cerut.
- Buton "Aplica" (POST /admin/set-tier): aloca plan real si INCHEIE trial-ul
  (efect imediat; altfel trial-ul Pro universal de 30z masca alegerea).
- Control "Trial Pro N zile" (POST /admin/set-trial via accounts.set_trial):
  acorda/prelungeste trial fara a schimba tier-ul de baza.

Teste: signup (consent obligatoriu, requested_plan persistat, tier ramane free),
panou admin (set-tier incheie trial, free opreste Pro imediat, set-trial, validari
+ CSRF). Call-site-urile existente POST /signup actualizate cu consent.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/schema.sql

@@ -32,6 +32,15 @@ CREATE TABLE IF NOT EXISTS accounts (
     tier            TEXT NOT NULL DEFAULT 'free'
                        CHECK (tier IN ('free','standard','pro','premium')),
     trial_until     TEXT,                       -- ISO datetime UTC sau NULL; nullable
+    -- Planul CERUT de client la signup (separat de `tier`). NU acorda drepturi:
+    -- `tier` ramane sursa unica de adevar pentru gate-ul API (require_api_access) si volum.
+    -- Folosit la integrarea platilor: client cere plan -> plateste -> admin/webhook urca `tier`
+    -- -> API se deblocheaza. NULL = necunoscut (cont creat via CLI / inainte de coloana).
+    requested_plan  TEXT
+                       CHECK (requested_plan IS NULL OR requested_plan IN ('free','standard','pro','premium')),
+    -- Marca temporala a acceptarii Termenilor + politicii de confidentialitate (GDPR, L.142).
+    -- Setata la signup (proba de consimtamant). NULL = cont fara flux de consimtamant (CLI/legacy).
+    consent_at      TEXT,
     created_at      TEXT NOT NULL DEFAULT (datetime('now'))
 );
 -- Un CUI = un cont (cand e prezent). NULL ramane distinct nativ in SQLite -> conturi