[P2][SOC-01] Unified feeds and cross-network normalization

Programme: #1
Epic: #28

## Objective
Create one user-owned social/feed workspace across supported networks while preserving each provider's identity, thread, visibility, moderation and deletion semantics.

## Scope
- Canonical post/thread/account records with original provider IDs and revisions.
- Multi-account/network feed ingestion through connectors.
- Deduplication/cross-post correlation without destroying original records.
- Explicit verified/uncertain cross-network identity links.
- Thread/parent/reply relationships and attachment metadata.
- Visibility/audience, labels/moderation, blocks/mutes and action availability.
- Tombstone/deletion/visibility-change propagation into derived views/indexes.
- Original/evidence navigation.

## Data rules
- Display-name/avatar similarity is never sufficient to merge identities.
- Normalized feed is a view, not the source of truth.
- Provider semantics remain attached to each record/action.

## Acceptance criteria
- [ ] Similar-name adversarial accounts remain distinct unless explicitly verified/linked.
- [ ] Deleted/private/blocked/muted provider records propagate appropriately into derived views.
- [ ] Cross-post deduplication retains access to each original source/network.
- [ ] Thread structure remains correct across refreshes.
- [ ] Account/network/visibility is visible for each item.
- [ ] Feed remains usable as a deterministic workspace without an active model call.

## Dependencies
- CONN-01
- DATA-01
- UI-02

**First phase:** P2
**Maturity target:** P5
**Owner:** connectors-domains

Start by reading the CONN-01, DATA-01, and UI-02 dependencies to understand the connector, data, and interface boundaries. Done means satisfying the listed acceptance criteria: preserving provider identity and semantics, propagating record changes, retaining original sources, maintaining thread structure, showing account/network/visibility, and keeping the feed usable without an active model call.

Written by the indexing model from the issue text.