Social auto-posting (phase 1): daily applicant digest to Telegram/Bale + Instagram caption

Adds a «شبکه‌های اجتماعی» admin section + scheduler that publishes a daily
«کادر آماده‌به‌کار امروز» digest:

- AppSetting: social toggles, posts-per-day, editable header/footer,
  per-channel bot token + chat id (Telegram, Bale), Instagram enable +
  extra hashtags, proxy toggle, last-posted timestamp (+ migration).
- SocialPostService: builds today's talent digest as text, posts to
  Telegram and Bale via their bot sendMessage APIs (proxy-aware), and
  produces an Instagram caption + auto hashtags (role/city based).
- SocialPostWorker: posts N times/day, evenly spaced, self-paced; reads
  settings live so it's togglable without redeploy.
- /Admin/Social: credentials + header/footer + posts/day, live preview of
  today's message, «ارسال اکنون» button, and an Instagram caption pack
  with copy button (semi-automatic — you post the image manually).
- Nav link added.

Telegram/Bale post as TEXT (per request). The Vazirmatn image card for
Instagram is phase 2 (needs SkiaSharp+HarfBuzz + a TTF font).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/JobsMedical.Web/Program.cs

@@ -59,6 +59,8 @@ builder.Services.AddSingleton<JobsMedical.Web.Services.Scraping.IListingSource,
 builder.Services.AddScoped<JobsMedical.Web.Services.Scraping.ListingArchiver>();
 builder.Services.AddScoped<JobsMedical.Web.Services.Scraping.IngestionService>();
 builder.Services.AddHostedService<JobsMedical.Web.Services.Scraping.IngestionWorker>();
+builder.Services.AddScoped<JobsMedical.Web.Services.Social.SocialPostService>();
+builder.Services.AddHostedService<JobsMedical.Web.Services.Social.SocialPostWorker>();
 
 // Phone-OTP cookie auth.
 builder.Services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)