feat(raptor): add Psi tree builder with original-space ranking and safe migration (#14679)

### What problem does this PR solve?

Closes #14674.

This PR improves RAPTOR configuration and tree construction while
preserving the existing RAPTOR behavior as the default.

RAPTOR currently builds summary layers with the original UMAP + GMM
clustering path. This PR keeps that default path, and adds:

- A hidden backend tree-builder option:
  - `tree_builder="raptor"`: default, existing RAPTOR behavior.
- `tree_builder="psi"`: rank-aware Psi-style tree builder using original
embedding-space cosine ranking.
- A user-facing clustering method option for the default RAPTOR builder:
  - `clustering_method="gmm"`: existing default.
- `clustering_method="ahc"`: agglomerative hierarchical clustering path.
- A RAPTOR UI setting for `Clustering method` and `Max cluster`.

### What changed

#### Backend

- Added `tree_builder` support for RAPTOR/Psi.
- Added `clustering_method` support for GMM/AHC.
- Kept existing RAPTOR + GMM as the default.
- Added Psi tree building from original-space cosine similarity.
- Added bucketed Psi building controls for large inputs:
  - `raptor.ext.psi_exact_max_leaves`
  - `raptor.ext.psi_bucket_size`
- Added method-aware RAPTOR summary metadata using existing
`extra.raptor_method`.
- Avoided adding a dedicated DB schema field for experimental method
tracking.
- Added cleanup/migration logic to avoid mixing stale RAPTOR summary
trees.
- Added defensive checks for Psi tree construction and summary failures.

#### Frontend/UI

- Added `Clustering method` in RAPTOR settings with `GMM` and `AHC`.
- Added/kept `Max cluster` in RAPTOR settings.
- Enlarged max cluster UI limit to `1024`, matching backend validation.
- Kept AHC editable even when a RAPTOR task has already finished.
- Fixed the UI save payload so `clustering_method` and `tree_builder`
are serialized through `parser_config.raptor.ext`, avoiding backend
validation errors for extra top-level RAPTOR fields.

Example saved RAPTOR config:

```json
{
  "raptor": {
    "max_cluster": 317,
    "ext": {
      "clustering_method": "ahc",
      "tree_builder": "raptor"
    }
  }
}

Co-authored-by: CaptainTimon <CaptainTimon@users.noreply.github.com>

web/src/locales/en.ts

@@ -861,6 +861,11 @@ The above is the content you need to summarize.`,
       thresholdTip:
         'In RAPTOR, chunks are clustered by their semantic similarity. The Threshold parameter sets the minimum similarity required for chunks to be grouped together. A higher Threshold means fewer chunks in each cluster, while a lower one means more.',
       thresholdMessage: 'Threshold is required',
+      clusteringMethod: 'Clustering method',
+      clusteringMethodTip:
+        'Select the RAPTOR clustering method. AHC can use a larger max cluster value, but may require more memory on large inputs.',
+      clusteringMethodGmm: 'GMM',
+      clusteringMethodAhc: 'AHC',
       maxCluster: 'Max cluster',
       maxClusterTip: 'The maximum number of clusters to create.',
       maxClusterMessage: 'Max cluster is required',

web/src/locales/zh.ts

@@ -772,6 +772,11 @@ export default {
       maxTokenMessage: '最大token数是必填项',
       threshold: '阈值',
       thresholdMessage: '阈值是必填项',
+      clusteringMethod: '聚类方法',
+      clusteringMethodTip:
+        '选择 RAPTOR 聚类方法。AHC 可以使用更大的最大聚类数，但在大规模输入时可能占用更多内存。',
+      clusteringMethodGmm: 'GMM',
+      clusteringMethodAhc: 'AHC',
       maxCluster: '最大聚类数',
       maxClusterMessage: '最大聚类数是必填项',
       randomSeed: '随机种子',