feat(dashboard): add validator score history view (issue #76)

[flo2517]

What

Reads pallet-network-validator's Rounds StorageNMap directly (never read from Go before this PR) and surfaces it as a new dashboard endpoint and UI panel — the first concrete piece of #76's validator-facing views, unblocked by #85's committee/evidence work.

internal/blockchainbridge/roundresult.go

• RoundResult/RoundStatus mirror the pallet's struct/enum exactly (fixed-width SCALE, no compact encoding), verified directly against blockchain/pallets/network-validator/src/lib.rs.
• roundResultStorageKey builds Rounds' 3-key StorageNMap key (Blake2_128Concat AccountId, Twox64Concat u64, Twox64Concat ScoreDimension) — this codebase's first NMap key; everything prior only needed the single-key mapStorageKey helper. Evidence uses the identical key shape if a future caller needs it.
• twox64 is twox128's 8-byte single-pass sibling (Twox64Concat's hash component).
• FinalizedRoundResult reads one (provider, round, dimension) point; found=false means the round hasn't closed yet — a normal state, not an error.
• ConfidenceBps turns submissions/committee_target into a display figure so a thin, low-confidence round can't be confused with a well-attested one.

internal/dashboard/validatorscores.go

• GET /api/v1/validator-scores/{provider_id}: for each of the 5 score dimensions, concurrently scans the last 12 rounds back from the current round (derived via the existing internal/networkvalidator.RoundLength helper) for closed rounds, returning up to 8 per dimension.
• Unauthenticated and rate-limited, matching agentEndpoint's reasoning — this is a narrower view over reputation data already public in /api/v1/overview, not a new trust boundary.

Dashboard UI

A new "Historique des rounds de scoring" panel — on-demand lookup by provider_id, not folded into the periodic overview poll, since each load is real per-round chain I/O.

Testing

• Unit tests for the storage-key construction, decode, and confidence math.
• Live-chain integration tests (OPENINFRA_TEST_SUBSTRATE_RPC_URL) proving the whole read path against the running local dev node, including the concurrent per-dimension scan end to end via the HTTP handler.
• As with every pallet-network-validator interaction this session, the local dev chain's wasm predates the pallet, so only the "not found" path is live-verified, not a real closed-round decode — documented in the test comments, not hidden.
• gofmt, go vet, go build ./..., and the full control-plane test suite all ran clean.

This leaves #76 open: operator views, RBAC, pagination beyond hardcoded LIMITs, accessibility review, E2E tests, and the decentralized-hosting migration doc are still outstanding.

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