Add VK (VKontakte) as a platform

[gynsus]

Why

VK (VKontakte) is the dominant social network for Russian-speaking audiences, and self-hosted TryPost users publishing there currently have to do it by hand. This PR adds VK as a full platform: connect, publish (text, photos, video), and analytics.

The connect flow is deliberately token-based, not OAuth. VK stopped granting the wall scope to newly registered OAuth apps years ago, so a stock "log in with VK" integration cannot post to walls at all. What still works, verified against the live API:

• a community access token (community → Manage → API usage → Access tokens) — wall.post with owner_id=-<group> + from_group=1 is allowed for these regardless of the app type that issued them; this is the recommended path and the one the form suggests first;
• a user access token issued by a standalone app the user owns (wall, photos, groups, video, offline scopes) — needed for profile walls and for video upload, which VK only allows with user tokens.

What changed

• Platform::Vk + ContentType::VkPost (16,000-char limit, VK's actual cap), lang strings for all 16 locales, platform tile/logo/theme.
• Connect (VkController): one token field for both token kinds. A community token is detected by a successful-but-empty users.get (the method accepts community tokens and just returns an empty list without user_ids — it does not error). Three live-API quirks shaped the flow:
  • VK has no API to tell which community a token belongss to, so for community tokens the form asks for the community address (vk.com/club…, screen name, or bare id — all normalized) and resolves it via groups.getById.
  • Ownership is then proven with groups.getCallbackConfirmationCode, which only answers for the token's own community. (groups.getOnlineStatus looked right for this but fails with "group messages is disabled" for communities that turned messages off.)
  • For user tokens, users.get + groups.get(filter=admin,editor) builds a picker of publishable walls (own profile + administered communities).
• Publish (VkPublisher): wall.post with attachments; photos via the three-step getWallUploadServer → upload → saveWallPhoto flow (images optimized first), video via video.save with wallpost=0 so the video attaches to our post instead of auto-publishing. A failed single attachment is skipped with a log, not a sunk post. VK reports errors as HTTP 200 + error object; VkPublishException maps codes to the shared error categories (rate limit / permission / media format).
• Analytics (VkAnalytics): community member count on the account level; views/likes/reposts/comments per post via wall.getById. Handles both the v5.199 response shape (response.groups[] / response.items[]) and the older flat one.
• Verification (ConnectionVerifier): users.get for user tokens, groups.getById for community tokens (which users.get can't verify); VK error 5 is the confirmed-dead-token signal.

Testing

• New suites: VkControllerTest (9 tests: both token kinds, community mismatch/unknown address, VK-side errors), VkPublisherTest, VkAnalyticsTest, VkPublishExceptionTest, plus ConnectionVerifierTest additions for both verify paths.
• Battle-tested on a self-hosted production instance: both token kinds connected, wall posts with photos published to two communities, analytics rendering member counts and per-post metrics.

What only you can decide

Token-based connect is unusual for this codebase — every other platform is OAuth. We kept the UI to the same two-step popup pattern as LinkedIn's identity picker, but if you'd rather gate VK behind a SELF_HOSTED-style env flag until it settles, that's a small change on top.

[paulocastellano]

@gynsus I'm looking into this; do I need a Russian VPN to install this app?

[gynsus]

Should work worldwide, no VPN needed — vk.com and api.vk.com are not geo-restricted. For the easiest test path you don't even need to register a VK app: any VK account (a non-Russian phone number should work for signup) → create a community → Manage → API usage → Access tokens → create a token with photos/wall/manage → paste it into TryPost's Connect VK form. That exercises the recommended community-token flow end to end. The user-token path needs a standalone app of your own, but it's optional — happy to walk you through it if you want to test video upload too.