feat: rich character profiles — brand voice, target, rules, hashtag profiles

Backend:
- Character model: add brand_voice, target_audience, business_goals,
  products_services, content_rules (JSON do/dont), hashtag_profiles (JSON)
- Content generation: inject full character context into LLM system prompt
  (voice, audience, goals, products, rules)
- Hashtag generation: merge always-on tags from profile with AI-generated tags
- Schema: update CharacterBase and CharacterUpdate with new fields

Frontend:
- CharacterForm: new sections "Identità e Voce", "Regole Contenuti",
  "Profili Hashtag" with dedicated editors
- RulesEditor: do/don't list with add/remove
- HashtagProfileEditor: per-platform tabs, fixed hashtags + max generated count
- All fields loaded on edit, saved on submit

DB migration: 6 new columns added to characters table

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas.py

@@ -24,6 +24,12 @@ class CharacterBase(BaseModel):
     niche: str
     topics: list[str] = []
     tone: Optional[str] = None
+    brand_voice: Optional[str] = None
+    target_audience: Optional[str] = None
+    business_goals: Optional[str] = None
+    products_services: Optional[str] = None
+    content_rules: dict = {}          # {"do": [...], "dont": [...]}
+    hashtag_profiles: dict = {}       # per-platform: {"instagram": {"always": [], "pool": [], ...}}
     visual_style: dict = {}
     social_accounts: dict = {}
     affiliate_links: list[dict] = []
@@ -40,6 +46,12 @@ class CharacterUpdate(BaseModel):
     niche: Optional[str] = None
     topics: Optional[list[str]] = None
     tone: Optional[str] = None
+    brand_voice: Optional[str] = None
+    target_audience: Optional[str] = None
+    business_goals: Optional[str] = None
+    products_services: Optional[str] = None
+    content_rules: Optional[dict] = None
+    hashtag_profiles: Optional[dict] = None
     visual_style: Optional[dict] = None
     social_accounts: Optional[dict] = None
     affiliate_links: Optional[list[dict]] = None